written upon it, infection is too much regarded as a simple and uniform act; and the virus transmitted as the same in quantity and intensity. Such views, however, carry error into every part of the discussion. If we can dilute the matter of small-pox, so that it is no longer capable of giving the disease by inoculation, equally may the effluvia of certain fevers, capable of communicating the disorder in one degree of concentration be so diluted in other cases either in their original emission from the sick body—or by distance — or from the state of the atmosphere- or by the intervention of other matters that they lose the power of reproducing the disease in its complete and specific form. Accordingly we find that in these fevers, as well as in diseases more undoubtedly and actively contagious, the proofs of infection multiply, in proportion as the causes exist which are likely to concentrate, or give direction to, infectious matter; as stagnant air, want of cleanliness and fresh clothing, proximity of place, particular currents of air, &c. And what is true as to these disorders, will equally apply to many other doubtful or anomalous cases in the history of disease.

In erysipelas, for example, though its occasional contagious nature is sufficiently proved, the instances of this are comparatively so rare that it occurs in the light of an anomaly to common observation. Nor can I doubt (having seen cases which go far to prove it) that a patient labouring under genuine measles may be present to another, perfectly susceptible of the infection, without the latter receiving the disorder; in default of the quantity or other peculiar state of the virus, needful for its passage through the intervening air. And this point receives further illustration from those singular cases, where an imperfect and irregular evolution seems to occur of an infectious disorder, the actual nature and presence of which cannot be doubted. These various conditions

can in no way be so well explained as by looking to the difference in quantity or concentration of a given virus; a diversity which must be of constant occurrence, and can never occur without some change of effect.

It is another and frequent mistake, in reasoning upon contagion, to consider that the infectious nature of a disease may be disproved, by showing that it has been spread without any obvious communication through man or human means. The two conditions brought into the question are in fact perfectly compatible with each other. If a virus can be transmitted from the body through a few feet of air, we are not entitled from the partial experiments hitherto made to set any limit to the extent to which, under favourable circumstances, it may be conveyed through the same or other medium. Common reason here concurs with our actual experience of the transmission of the virus of certain diseases in various ways, and to remote distances.

Notwithstanding all the labours of the time past, the subject of contagious disease still offers a spacious field for discovery — perhaps more extensive and curious than any other part of medical science. In the remarks just made I have sought merely to remove two or three common errors, and to put into a precise form those considerations or methods which must, I think, be the groundwork of all inquiry on the subject; purposely abstaining from any but the simplest illustrations, though others might be drawn from all parts of pathology. The methods suggested will apply to every view of the nature of the virus of particular diseases; and, under the scanty amount of our actual knowledge, this general application is, perhaps, the best test we can have of their value. Neither the laws of contagion, nor the many collateral questions which have perplexed and angered medical authors, can be rightly settled without some common principles to

which to refer for classification of facts, and guidance amidst the seeming anomalies they present.

* A valuable paper on the laws of contagion, by my lamented friend Dr. Henry, is contained in the Fourth Report of the British Association, in which will be found the notice of nearly all that forms our present knowledge on the subject.

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CHAP. XIX.

ON THE MEDICAL TREATMENT OF OLD AGE.

THERE are some points belonging to the disorders and treatment of old age, which, though familiar to the experienced physician, are scarcely enough regarded in general practice. It is understood that the rules which apply to other periods of life require alteration here, but the causes and extent of this are not always considered. True it is that old age is not to be reckoned merely by number of years. Family temperament, individual constitution, and the incidents of life, all concur to modify the time at which those changes begin which warrant the term in a physiological sense; and to affect no less the rate at which they proceed. Old age, moreover, may be said to be unequal in different individuals, as respects the different parts and functions of the body; this diversity arising out of the same general causes which have just been assigned. Hereditary malady, or incidental disorders, often bring on premature decay in one organ, while others are comparatively untouched by time. These are points of importance to the practitioner; but withal so complicated, as scarcely to admit of being classed or described. Experience alone can adequately teach them.

Taking the term of old age, however, in its familiar sense, there is no difficulty in recognizing certain changes that have already taken place, or are still proceeding, in all parts of the animal economy; nor doubt as to the fitness of modifying medical treatment, whether preventive or curative, with reference to these.

Bichat has defined life, "l'ensemble des fonctions qui résistent à la mort;" and the description, however quaint (and perhaps not strictly logical), is true in one important sense; viz. that living organization is that which is withdrawn for a time from what we understand as the ordinary laws and conditions of matter, and seems in many points even placed in opposition to them.* I use general terms here, because I believe that our knowledge does not justify our drawing any distinct line between what have been severally named physical and vital laws. Such line may exist; but the attempt to define it at present rather marks our own ignorance, than any natural boundary between the laws which govern organic and inorganic creation. All great discoveries in physical science (and especially some of recent date) have had the effect of altering this presumed boundary, and, for the most part, of extending the domain of physical into that of vital phenomena. †

* A grosser definition of life, though in the same sense, is given by another author, “ Illud putredini contrarium." It is obviously easy to multiply definitions founded on this principle of description; and many in truth might be cited, which have no other merit than the same paradoxical affirmation of some partial truth.

+ Chemistry in its largest sense, connected on one side with the great mechanical laws which govern the universe; on the other, with those imponderable agents, light, heat, and electricity, which science is now fast submitting to the same laws, is apparently the source whence we may expect the greatest and most certain results.

It is needful here, however (and especially for the student or general reader on such subjects), to understand the true bearing of these, even if carried far beyond their present limits. They do in nowise alter or impair the evidence of Natural Theology, but even enforce and enlarge it. If an agency, wholly unknown to us before but in a certain series of effects, is proved to be identical with one producing, under other circumstances, a different series, we extend in our comprehension the scope of that power which can govern and direct the same agents to such vast and various ends; and we establish yet further the unity of that design which pervades, associates, and sustains the whole.