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Endothermic Decompositions obtained by Pressure.

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Apart from the comparison of the specific gravity of the ethyl acetate before mixing and after fractionation, a proof of the purity of this substance is afforded by the agreement between the specific gravities of the two samples, since, if the separation had been incomplete, one would have been contaminated with methyl acetate, the other with propyl acetate.

III. On Endothermic Decompositions obtained by Pressure. (Second Part.) Transformations of Energy by Shearing Stress. By M. CAREY LEA*.

OF

F the relations which exist between two forms of energy, mechanical and chemical, very little, if anything, is known. In the second volume of his Lehrbuch Ostwald remarks that as to these relations "almost nothing" is known t.

There are certain familiar cases in which mechanical energy may seem at first sight to be converted into chemical energy. The fulminates, iodimide, and other substances explode by shock. But it is hardly necessary to remark_thatall such reactions are exothermic, and need an external impulse only to start them-if this impulse were not needed such compounds could not exist at all. Were such reactions taken as true transformations of energy, an absence of due relation between cause and effect would be involved; for the shock that suffices to explode a grain of fulminate will equally explode a ton, and the faint spark that will explode a grain of gunpowder will also explode a magazine.

Present opinion holds undoubtedly that no true transformation of mechanical into chemical energy is known. Most text-books do not consider the question at all. But Dr. Horstmann, in the volume of theoretical chemistry which forms part of the last German edition of Graham-Otto's Chemistry,' discusses the matter. His views are so much to the point that I shall translate a few sentences, putting in italics the statements to which I would specially refer.

"We must consequently admit that through a rough me

chanical attack the molecular structure of certain chemical compounds can be disrupted and destroyed. This will certainly be possible only for compounds in whose molecules the arrangement of the atoms does not correspond to a stable *Communicated by the Author.

"Anderseits ist von dem Verhältniss zwischen mechanischer und chemischer Energie fast nichts bekannt." A few lines below this remark is repeated with emphasis. Lehrbuch, 2nd German ed. vol. ii. p. 12.

equilibrium, and in which, therefore, the chemical energies themselves are already striving to form simpler and more stable compounds out of the constituents of the existing substance. For it cannot be admitted that actual chemical changes can be brought about by a mechanical impulse" (1. c. p. 350). In another chapter he says with equal distinctness :

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By mechanical means alone no reaction against the force of chemical energy can be brought about. By a shock or blow the molecular structure of chemical compounds can indeed be so far loosened that free play is given to chemical forces; but against these forces we cannot by mechanical means separate the atoms nor combine them in a definite way" (p. 594).

These expressions of a distinguished chemist will sufficiently indicate what has been up to the present time the opinion of chemists as to the possibility of transforming mechanical energy into chemical.

In the first part of this paper I believe I have been able to show in a qualitative way the production of true endothermic reactions by mechanical force. In the present part I hope to show an increased number of such reactions, and in one case to exhibit actual quantitative results, at least so far as to obtain the product of the transformation in weighable quantities.

In the first part decompositions were described that were brought about by simple pressure. Compounds formed by exothermic reactions, and therefore requiring expenditure of energy to break them up, were decomposed. The investigation might probably have been made to include a still larger range of substances. But it was found that the efficiency of pressure was so enormously increased by the addition of shearing motion, that decompositions requiring a force of hundreds of thousands of pounds with pressure alone could be effected by the mere strength of the hand when shearingstress was used. More than this, decompositions which enormous pressures failed to effect readily took place under the action of shearing-stress*.

*It would not have been difficult to obtain much greater pressures than those described in the first part of this paper. This could be effected by means of the differential screw. I had planned for a screw with threads of 40 turns in 10 inches and 391 turns in 10 inches respectively. The mechanical efficiency of such a screw is that of one having 320 turns to the inch, if such a thing were practicable, at the same time that a thoroughly strong construction can be obtained. The massive steel nut to advance of an inch would require 40 full turns of this screw.

This arrangement compares as follows with that previously employed. In it to cause the vise-jaws to approach by 1 inch required that the point of the lever at which the force was applied should pass through a space

Decompositions obtained by Pressure.

I.

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It was mentioned in a previous paper on the decomposition of the silver haloids by mechanical force that when silver chloride was sharply ground for some time in a mortar, both the pestle and mortar became covered with a deep purple varnish of silver photochloride, thus indicating a partial reduction to subchloride. It has since proved that there is no more effectual method than this of applying shearing-stress, and that in this way a number of quite stable chemical compounds formed by exothermic reactions can be broken up. The mortar and pestle should be very solid and of unglazed porcelain. With metal there would be danger of action between the metal and the substance, and with agate mortars sufficient force cannot well be applied. In many cases success depends on the exertion of great pressure on the pestle. is also absolutely essential that the quantity of material acted upon should be small. When a larger quantity is employed the particles slip or roll over each other and thus escape the action of the stress. It is no doubt for this reason that the very remarkable results which can be obtained in this way have hitherto escaped attention.

It

A small quantity, a few decigrams, of the substance having been placed in the mortar, the first thing is to spread it in a thin uniform coat over the bottom and part of the sides. The pestle is then to be rotated with the utmost force that can be exerted.

Sodium Chloraurate.-The salts of gold are particularly well adapted to this examination, as the reduction is complete and the gold appears in the metallic state so that it can be weighed and the exact amount of reduction can be fixed. It

of 113.1 feet: this relation, 1 inch to 113.1 feet or 1:1357.2, gives the measure of the efficiency of the instrument.

With the double screw, on the other hand, to cause the nut to advance of an inch, the end of the lever (2 feet long) must pass through a space of 500 feet, or in the proportion of 1 inch to over of a mile. The circumference described by the lever being approximately 12 feet, and the screw requiring 40 turns to advance the nut of an inch, we get the proportion of 1 inch to 4000 feet, or 1 to 48,000, which is the measure of the efficiency of such an instrument. Therefore, supposing two men to pull on the end of the lever each with a pull of 100 lb., the pressure exerted on the nut (disregarding loss by friction) would be 9,600,000 lb., which could be doubled by using a 4-foot lever.

Such a combination is quite practicable, the only real difficulty being to obtain sufficient solidity of construction to resist strain. I had made drawings for this instrument, but gave it up in consequence of observing the enormously greater efficiency of shearing-stress as a means of transforming mechanical into chemical energy.

Phil. Mag. S. 5. Vol. 37. No. 224. Jan. 1894.

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will be seen by (3) below that it may amount to as much as over 4 per cent. of the gold present.

(1) Two or three decigrams of chloraurate with a moderate trituration left 1.8 milligram of metallic gold. Under the action of the pestle the yellow colour of the salt gradually deepened to an olive shade. When water was poured on, the undecomposed salt dissolved, leaving the gold as a delicate purple powder. The colour of the gold being purple instead of the more usual brown shade explains the olive colour just mentioned, yellow and purple combining to form olive.

(2) Half a gram of the salt was taken. This specimen was more neutral than the preceding, and was therefore more easily reduced. Half an hour's trituration had for effect the reduction of 9.2 milligrams of gold.

(3) A similar treatment of the same quantity of chloraurate resulted in the separation of 10-5 milligrams of gold.

These may seem at first somewhat small proportions. But it is to be recollected that the force is necessarily applied at a disadvantge, and that the equivalent in work of chemical affinity is always very large. In the present case the figures are as follows:-Thomsen found as the heat-equivalent for the combination of gold with chlorine to form auric chloride 28.8 great calories. Taking the atomic weight of gold as 197, we find that one gram of gold in forming auric chloride disengages 115 7 small calories or water-gram-degrees, whose equivalent, taking Rowland's determination, is 49,288.2 gram-metres, corresponding to 4.83 x 10° ergs or 483 joules.

The small quantity of gold reduced in (3), 10.5 milligrams, would by conversion to auric chloride generate 1215 watergram-degrees of heat whose equivalent in work is 518 grammetres. As heat is a degraded form of energy, such an actual transformation without loss to a higher form would be impossible. It is more correct to say, therefore, that the amount of energy which would raise 518 grams to the height of one metre can be transformed into the same amount of heat, 1.215 water-gram-degrees, as is evolved by 10.5 milligrams of gold by conversion into auric chioride. Consequently this work, 518 gram-metres, represents the amount of mechanical energy transformed into chemical energy in operation (3)*.

It does not appear that in effecting these reactions and the

The amount of energy required would, in fact, slightly exceed this, as the thermochemical equivalent of formation of sodium chloraurate would slightly exceed that of auric chloride. For this chloraurate I do not find a determination, but preferred to use this salt in the operation as being both more stable and more neutral than auric chloride.

others which remain to be described, mechanical energy undergoes an intermediate conversion into heat. Rapid movements are not needed; what is required is strong pressure with movement, but this need not be rapid. Nor does the mortar or the pestle become sensibly warm. The operation does not need to be continuous, but may be broken up with any number of intervals. But a decisive conclusion can be drawn from those cases in which decompositions are effected in this way that cannot be produced by heat. For example, in the next instance to be mentioned there is a partial reduction of corrosive sublimate to calomel. By heat, corrosive sublimate sublimes unaltered, and the same is true of mercurous chloride. The three silver haloids fuse unchanged at a red heat. The same conclusion can be drawn from other reactions.

These results were obtained in an atmosphere absolutely free from dust, so that the reducing action of this substance was completely excluded.

Mercuric Chloride.-A specimen which, after lightly powdering, did not darken in the least with ammonia, was triturated in the manner just described with several intervals, in all for 15 minutes. It then became grey in a very striking way when moistened with ammonia.

This is a very interesting reaction. In the first part of this paper it was mentioned that mercuric chloride could be subjected to a pressure of about 70,000 atmospheres absolutely without change. It appears, however, that a pressure amounting to less than a hundred pounds causes decomposition when combined with movement, showing the enormously greater efficiency of shearing-stress as compared with simple pressure. Not only this but, as just mentioned, shearingstress produces decompositions which heat is not competent to effect.

Mercurous Chloride.-When calomel was sharply triturated in a mortar, it first became yellow and then blackened without difficulty.

Turpeth Mineral, 3HgO,SO,.-Reduces rather slowly.

Mercuric Oxychloride, 2HgO, HgCl2, obtained by precipitating corrosive sublimate by potash acid carbonate, exhibited the following reaction. Its brownish-purple colour by light grinding became lighter, and then when strong force was used it blackened with remarkable ease.

Mercuric Iodide shows a trace of blackening.

Mercuric Oxide.-This substance yields much more readily to trituration than to simple pressure. Especially on the sides of the mortar it was quite blackened. The layer of material must be quite thin, otherwise little effect is produced.

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