278 Recent Progress in Sanitary Science. 



There is one more popular delusion, the overthrow of 

 which is to be ranked as an onward step in sanitary science. 

 This delusion is, that the senses are trustworthy sentinels 

 over our lungs and stomachs, and that dangerous air, water, 

 or even food, is always detected by them. They only serve 

 as detectives when one of the concomitants of aerial, aqueous, 

 or other filth is of the nature of a gas, like hydrosulphuric 

 acid, affecting, even when present in very minute quantities, 

 the sense of smell, or is one of the innumerable products of 

 organic decay. But the cases must be rare indeed, in which 

 fatal effects have been produced by exposure to an atmo- 

 sphere containing hydrosulphuric acid sufficiently concen- 

 trated to act as a chemical poison ; and although headache, 

 nausea, or a general lowering of the health, is frequently 

 produced in the case of persons occasionally exposed to a 

 considerable amount of this and similar gaseous products of 

 decomposition, or constantly inhaling them in minute quanti- 

 ties, yet they are comparatively harmless when compared with 

 some emanations which are not evident to the sense of smell. 

 The matter which propagates disease, is, so far as we 

 know, not gaseous, but organized bodies of excessively mi- 

 nute dimensions, so small indeed that as yet the microscopist 

 has not succeeded in distinguishing the "spores" which sim- 

 ply produce decomposition, from those which carry the spe- 

 cific poison of certain diseases, or the infectious germs of 

 one disease from those of another. But one peculiarity they 

 possess in common, a peculiarity distinguishing them from 

 chemical poisons, in that their effect is not directly propor- 

 tional to their amount, but vastly greater, insomuch that ex- 

 cessively minute amounts of these germs have the power of 

 infinite self-multiplication, so long as they find themselves 

 surrounded with circumstances favorable to their develop- 

 ment. It frequently is the case that localities are obnox- 

 ious in odor, yet no alarming diseases are developed, while 

 others are apparently inoffensive, and at the same time are 

 richly productive in "zymotic diseases." A striking illustra- 



