SciENiiFTC Lectures. 257 



muscular exertion is attended by a copious flow of ordinary fluid 

 perspiration. The system attempting in this manner to throw off 

 the heat resulting from the oxydation required to produce the muscu- 

 lar effort, on another day M-hen the air is dry, or the amount of 

 moisture far below the point of saturation, great muscular effort may 

 produce but little visible perspiration, even though the thermometer 

 indicates a higher temperature. In this case vaporization from the 

 body has been copious and sufficient to carry off' the heat as fast as it 

 was produced, and the full cooling effect of the process has been 

 realized. 



Snlphureted hydrogen is the most offensive to the senses of all 

 the components of the air. Like the other gases and vapors we 

 have examined, it is colorless, possessing an odor which may be 

 described as resembling that emitted by an Qgg in an advanced state- 

 of putrefaction. This property, by indicating its presence, generally 

 protects us from its evil effects; and even wlien the proportion is 

 too minute to affect the sense of smell, we may still detect it by its 

 action on various metallic compounds, some of which, as the carbo- 

 nate of lead, are employed in the painting of wood work. In illus- 

 tration of this property, the gas, as it is evolved from a suitable 

 apparatus, is passed through various solutions of metallic salts, when, 

 as you perceive, the metallic sulphides are thrown down, or, in 

 chemical parlance, precipitated as solids of different and sometimes 

 brilliant colors. In tlie case of the lead salt, the precipitate is 

 black, explaining to us the darkening that occurs in lead paints when 

 they are exposed to the continued action of this gas. 



In its physiological relation snlphureted hydrogen is a narcotic 

 poison, and endowed with energy, even when the proportion is very 

 small. It acts on the iron of the blood discs, darkening it as it did 

 the iron salt in yonder vessel, and, destroying the power of the disc 

 to perform its function, produces a cumulative effect, as was the case 

 with carbonic oxyd. 



The corpuscles or floating cells in the air are of many different 

 kitids. Some are the germs of cryptogamic and other lowly forms 

 of plant life, and there is but little doubt that many of the diseases 

 called contagious are conveyed by corpuscles or germs whicli attach 

 themselves to the motes that we see dancing in the path of a sun- 

 beam and freight them with a poison which, when it is introduced 

 into the respiratory apparatus of some unfortunate creature, gene- 

 rates the disease from which it was born. 



[Lnst.] 17 



