temperature in the evening without any long rest in the darkness, and, as is seen above, 17 minutes in perfect darkness produced no alteration; yet, in the morning, he went up to about the usual figure, so that what I had hoped to find an abnormal case turned out approximately ordinary.

The loss of distinct colour at the low temperatures is very striking; the appearance to myself, and to most of the observers, has absolutely nothing of red in it, but is like a white mist-the nearest comparison I can make.

In the morning observations, however, when the strip disappeared at from 460° to 470°, the last appearance was distinctly reddish; and this agrees with one observation noted at night, when after getting the visibility critical-point at about 390° C., the temperature was raised until one could declare for certain that the light looked red: it was then found to be 449°.

Of course, in all the observations, the luminous area was most distinctly seen by somewhat averting the gaze from it; generally I found it best to look in the direction of either far upper corner of the enclosure.

As already mentioned, most of the observers pronounced the appearance at the critical-point to be that of a "whitish mist;" one, however, thought he saw a slight "lilac tinge in it; and "Case G" declared it to be decidedly yellow, which is interesting, because to him a red mark on white paper (such as a pip on a card belonging to one of the red suits of a pack) appears yellow, by artificial light at night.

In one experiment a plate of glass, inch thick, and in another a layer of water, inch thick, were inserted between the strip and the eye, without making the slightest difference in the phenomenon; showing (1) that the point where these substances begin to be more or less opaque to infra-red radiation had not been reached; (2) that the small difference in intensity produced by their insertion had no appreciable effect. This last conclusion is far more strongly borne out by the equality of temperature in the case of the bare metallic and the black surfaces, and indicates that in all the cases it was wave-length, and not intensity, which was determinative of visibility, so disposing of the possible objection that the difference between "morning" and "evening might be due merely to the state of enlargement of the pupil of the eye, which would naturally be more contracted at the one time than at the other, thus affecting the total amount of radiation falling on the retina. Also, if such an objection were valid, it would imply that fatigue of the muscles of the

iris produced a relatively enormous "time-lag" in following changes of luminous intensity, which we know does not exist.

There seems, in fact, to be little doubt that the difference is due to the retina itself becoming sensitive to long waves after rest, which were incapable of affecting it when it was in some way fatigued by exposure to the ordinary bright light of day.

The next and obvious step is to find the respective wavelengths corresponding to the different temperatures. This point, however, and others, cannot be determined without some additions to the present apparatus, and will form the subject of a future paper.

LV. New Method of Determining the Relative Affinities of certain Acids. By M. CAREY LEA*.

THIS

HIS method of measuring affinities is based on the principle that the affinity of any acid is proportionate to the amount of base which it can retain in the presence of a strong acid selected as a standard of comparison for all acids. The standard acid being in all cases kept exactly at the same dilution.

An example will make this clearer. Sulphuric acid is here taken as the standard, and its presence or absence in the free state is ascertained by means of the herapathite test (described in this Magazine for July 1893). For simplicity, we will suppose that the quantity taken is always a grammolecule at a fixed rate of dilution. It is evident that two gram-molecules of sodium hydroxide would exactly saturate it. If, now, we take a given acid, we may find that a quantity of its sodium salt corresponding to three grammolecules of sodium hydroxide will exactly extinguish the reaction of a gram-molecule of free sulphuric acid. With still another acid we may find that a quantity of its sodium salt corresponding to four gram-molecules of sodium hydroxide is needed to extinguish the sulphuric reaction. Then the affinity of the second acid is exactly twice as great as that of the first. At the point where the free sulphuric-acid reaction was extinguished, the second acid under examination retained twice as much sodium as the first, and this quite independently of any question of comparative basicity.

Throughout the series of determinations here to be described the sulphuric acid was used invariably at the same degree

*Communicated by the Author.

Phil. Mag. S. 5. Vol. 37. No. 229. June 1894.

2Q

of dilution, otherwise the results would not be strictly comparative. Having obtained normal acid by titration with

N

pure sodium carbonate, this was further diluted to and 50

8'

to 100 cub.centim. were found a convenient quantity to employ. The salt to be tested was finely powdered and thoroughly dried at 100°, or at whatever higher temperature it could support. It was then placed in a weighing-bottle and cooled in a desiccator and kept there except for a few moments at a time. By using the dry salt, the dilution of the acid was kept constant. When the point of extinguishment seemed to be reached, at least four final crystallizations were made. Great care is necessary to seize the exact point of extinguishment. The quantity of the salt found is then reduced to correspond with one gram-molecule of sulphuric acid. It is next divided by its own molecular weight: this gives the number of molecules of the salt needed to extinguish the reaction in one molecule of sulphuric acid. In order to make it possible to compare acids of different basicities, the figures thus obtained must next be modified to correspond with the basicity of the acid used. If the acid is bibasic, no change will be needed. If monobasic, the figures obtained must be divided by 2. If tribasic, they must be multiplied by &c. Finally, as the quantity characteristic of the acid is the excess of the quantity found over the amount equivalent to one molecule of sulphuric acid, unity is deducted from the amount obtained, and the residue thus found represents the comparative affinity of the acid, and may be called its index.

This may be rendered more clear by one or two instances. In the case of hydrochloric acid, there was needed as a mean of many determinations 29.37 gram-molecules of sodium chloride to extinguish the reaction in one gram-molecule of sulphuric acid. At this point the solution necessarily contained

Na2SO,+2 HCl + 27.37 NaCl.

This is proved beyond question by the fact that the solution no longer gives a trace of reaction of free sulphuric acid. The quantity of 27-37 gram-molecules of sodium chloride is the proportion of undecomposed sodium chloride that must remain in the solution in order that the sulphuric acid may be completely converted into sodium sulphate, and may remain as such in the solution in a condition of equilibrium.

This number 27-37 therefore represents the strength of the affinity of hydrochloric acid for sodium. But in order to compare acids of different basicities it is convenient to refer

them all to bibasic sulphuric acid, and therefore the number just found must be divided by 2. Therefore 13.68 may be taken as the index of the affinity of hydrochloric acid, in comparison with those of other acids determined in like

manner.

Similarly with pyrophosphoric acid. The mean value found for the quantity necessary to extinguish the free sulphuric acid in one gram-molecule of sulphuric acid was found to be 0.963 gram-molecule of sodium pyrophosphate. At this point the liquid contains

Na2SO4 + (HP2O1)+·463(Na ̧Ð1⁄207)

in equilibrium. The number 463 therefore represents the comparative affinity of pyrophosphoric acid, except that as the acid is quadribasic the number found must be multiplied by 2 in order to bring it into comparison with bibasic acids. Therefore the index of pyrophosphoric acid is 926. This acid being quadribasic, half a molecule contains the quantity of sodium requisite to saturate a molecule of sulphuric acid, and therefore only half a molecule of pyrophosphoric acid is set

free.

In other words: It is found by experiment that the quantity of sodium pyrophosphate necessary to extinguish the reaction for free sulphuric acid with one thousand molecules of that acid, is 963 molecules; out of this, five hundred molecules of pyrophosphoric acid are set free as just mentioned, and there remain 463 molecules of undecomposed pyrophosphate. This number 463 multiplied by 2 because of the basicity of the acid, and divided by 1000 to make it correspond to one molecule of sulphuric acid, gives 926 as the index of pyrophosphoric acid.

The state of equilibrium is always conditioned by the degree of concentration. If to any solution of sulphuric acid a salt is added in just sufficient quantity to extinguish the sulphuricacid reaction, it is then only necessary to add a little water and the equilibrium is at once changed; a certain portion of the salt that had been added is re-formed and the sulphuric reaction reappears. In order, therefore, to obtain true comparative results, it is necessary to use the sulphuric acid always at exactly the same dilution and to add the dry salt to it.

The affinity of sulphuric acid for water is a most important factor in all determinations of this nature. Mendeléef indeed expresses the opinion* that most of the affinities hitherto

*Principles of Chemistry,' English edition, vol. i. p. 377, footnote.

determined are unreliable for want of sufficient exactitude in this respect.

To show how much precaution is needed the following reactions may be mentioned.

When 4 cub. centim. of normal sulphuric acid are added to 40 cub. centim. of normal solution of sodium nitrate, not a trace of free sulphuric acid can be detected in the liquid. In consequence of the large excess of sodium salt the sulphuric acid has been completely taken up by the sodium, with of course an expulsion of an equivalent quantity of nitric acid.

But when, instead of 4 cub. centim. of normal sulphuric acid we use 40 cub. centim. of decinormal acid, then, although the quantities of acid and of salt are exactly the same, the equilibrium is completely changed. The greater quantity of water present by reason of its affinity for sulphuric acid counteracts to some extent the affinity of the sodium. Free sulphuric acid exists in the solution, and is abundantly indicated by the herapathite test.

This difference may be even more strikingly shown in the following manner :-Taking the mixture of 4 cub. centim. of normal sulphuric acid and 40 cub. centim. of normal solution of sodium nitrate, let a drop be placed in each of two small porcelain basins previously slightly warmed. To one of them let a single drop of distilled water be added, and then the herapathite test to both. In a few minutes the one which has received the drop of water will show well-marked crystallizations of herapathite, whilst the other will not show a trace. The effect of dilution in changing the equilibrium of the solution of a base with mixed acids is thus made visible to the eye by a chemical reaction. Hitherto it has been a deduction from physical changes requiring great delicacy of measurement. Results of a precisely similar character were obtained when potassium bromide was substituted for sodium nitrate, and are no doubt of general occurrence.

The applicability of this method proved to be a good deal restricted owing to the tendency of many acids when set free to decompose the herapathite reagent. For this reason the affinities of hydrobromic, hydriodic, chloric, iodic, and nitric acids could not be measured with accuracy, although many attempts, sometimes as many as 30 or 40 or more, were made to get reliable results. This work, however, was not entirely thrown away. It demonstrated that chloric acid has the strongest affinity for bases of any known acid. It might have been expected à priori that a highly oxidized acid of chlorine would have stronger affinities than chlorine hydride.