71. FOURTH REPORT — 1834. 



that disease*. But the liquid poisons, dried at the lowest tem- 

 perature adequate to that purpose, may be kept in close vessels 

 unimpaired for an indefinite time, and regain their infectious 

 properties when moistened with very little water. The mixture 

 of them, however, with a large proportion of water, renders them 

 inefficient. Dr. Darwin relates that, in some experiments by 

 Mr. Power, smallpox matter was found to be infectious after 

 diffusion through five times its quantity of water; but that its 

 dilution might be can-ied so far as to render it inertf. This is 

 precisely analogous to what happens with common poisons, 

 the most virulent of which is disarmed of its noxious power, 

 when sufficiently diluted, 



XII. Of the chemical constitution of the liquid contagious 

 poisons we are entirely ignorant ; nor is it probable that the 

 knowledge, if we possessed it, would throw any light on their 

 mode of action. We are well acquainted with the composition 

 of many poisons (the prussic and arsenious acids, for example), 

 without at all understanding in what way they act so powerfully 

 upon the animal system. 



XIII. Beside the liquid poisons, requiring contact for their 

 operation, there is another class which are independent of that 

 mode of communication, and are transmitted to small distances 

 through the atmosphere. Such are those of scai-latina, measles, 

 hooping-cough, chicken-pox J, &c. In a few instances diseases 

 imparted by contact are also caught by emanations or effluvia. 

 The smallpox, it is well known, may be propagated in both 

 ways ; and the plague, certainly infectious at small distances, 

 has, of late years, been proved to be communicable by inocula- 

 tion with the matter of the glandular abscesses. Dr. White, 

 after two unsuccessful attempts to inoculate himself, caught the 

 plague by the third, and died in three days; and Dr.Valli, in 

 180.S, fell a victim to a similarly rash experiment §. 



* Jenner's Further Observations, p. 19. f Zoonomia, u. s. 



X Cliickcn-pox (varicella) is not inoculable. See Thomson's History of 

 Smallpox, 8vo, p. 283. 



§ See Sir Robert Wilson's History of the Expedition to Egupt, p. 257; 

 •Wittman's Travels in Turkey, pp. 516, 518 ; and Granville in the Pamphleteer, 

 XXV. About the close of the sixteenth century a dispute arose, which has conti- 

 nued almost to the present day, whether the plague be a contagious disease or 

 not. Exclusion from that class has been extended also to typhus, yellow fever, 

 and scarlatina. Indeed smallpox and measles are the only febrile maladies, 

 which are admitted by some of the opponents of contagion to be propagated by 

 a specific poison. All others, affecting numbers at one place and one time, have 

 been by them classed with epidemics. It is needless to reply to the arguments 

 in favour of this doctrine, because they have been already refuted, in a man- 

 ner that should set the question at rest fqr ever, by Dr. Roget, in a Report 

 presented to Parliament in 1825. {^lee Parliamentary History and Review, 



