REPORT ON THE LAWS OF CONTAGION. 87 



nieratioii of which it is needless to repeat. Dr. Russell observed 

 the plague to "hang ambiguously" for several days about per- 

 sons. In this state, and even when there was no such evidence 

 of being infected, an overheated bath or a sudden impression of 

 fear, especially when the disease itself was the object, has ex- 

 cited the lurking poison into activity*. The late Dr. Jenner, 

 after having been much exposed to typhus contagion, experi- 

 enced no ill effect until a long and fatiguing ride on horseback 

 in extremely cold weather proved an exciting cause of that ma- 

 lady, which he then underwent in its usual formf. Dr. Lind 

 relates, that out of a number of sailors, all of whom had been in 

 the way of febrile infection, a part only, who had been permit- 

 ted to go ashore, and while there had been engaged in a debauch, 

 fell sick of low fevers. 



XXXVII. Among causes influencing the spread of contagious 

 diseases, climate has been reckoned, using that term in its en- 

 larged sense, and not merely as applied to geographical position. 

 There can be no doubt that climate modifies the jjredisjjosition 

 of the human body to receive infections. In addition to this ef- 

 fect, varieties of temperature, one of the principal elements 

 of climate, must necessarily affect the elasticity of vaporous con- 

 tagions, and consequently their diffusibilities. Certain poisons 

 (those perhaps which appear to have low vaporising points, as 

 smallpox, influenza, and Asiatic cholera,) exert their powers 

 alike in the hottest and coldest regions. Other poisons demand 

 a temperature not below 60° of Fahrenheit's thermometer]:. 

 Such is that of plague ; while the yellow fever does not exist at 

 temperatures below 80°, and in North America has been checked 

 in its spread by a single frosty night. But an increase of tem- 

 perature above a certain point (90") disarms the contagion of 

 plague of its power§ ; and typhus (or hospital) fever is unknown 

 in tropical regions||. Measles and scarlatina also are, in such 

 countries, of very rare occurrence. It is not improbable that 

 the highest temperatures observed in the atmosphere may ac-^ 

 tually destroy or decompose contagious poisons, as I have en- 

 deavoured to prove may be effected, so far as respects those of 

 cowpox and scarlatina, by temperatures not greatly exceeding 

 100° Fahr.^. 



The influence of weather over the spread of contagion has not 

 been sufficiently examined. So far as respects predisposition, it 



* Howard, p. 33 ; and Russell, p. 303. f Baron's Life of Jenner, p. lOG, 



X Blane, Med. Log., p. 173. § Russell, Antes, &-c. 



II Dr. Hunter, Medical Transactions, vol. iii. p. 355. 



H Phil. Mag. and Ann. ofPhilos., November 1831, and January 1832. 



