Scientific Lectures. 255 



peratnres, it allows its ^^assage like water tlirougli a sieve wlien the 

 temperature is raised to a bright red heat. Usually this is the condi- 

 tion of a stove or furnace in the winter season, and it necessarily 

 follows, that whenever carbonic oxyd is produced, it must escape 

 through the iron and gain access to the air of tlie apartment, unless 

 we employ a proper lining of soapstone or some material which shall 

 keep the metal at a lower temperature. 



The next gas to which your attention is called is ozone. Its prop- 

 erties are similar to those of oxygen ; and it is in reality a modified 

 or active form of that element, possessing the power of uniting with 

 many substances at the ordinary temperatures of the air, while oxy- 

 gen, as we have seen, requires the lieat to be raised to the point of 

 ignition. Its presence is determined by its action upon paper that 

 has been dipped in an aqueous solution of starch and iodide of pot- 

 assium — this it turns from a white to a blue or brown color, accord- 

 ing as it is more or less concentrated. Owing to its active oxydizing 

 power, ozone is a valuable disinfectant, since it can decompose nox- 

 ious gases and vapors, converting them into harmless bodies ; and 

 there is good reason for supposing that the purity of the air, after a 

 thunder storm, is in part owing to the conversion of a portion of its 

 oxygen into ozone, and the consequent removal of the offensive 

 ingredients. It has been stated that some epidemics, of which chol- 

 era is an example, reach their point- of greatest malignancy when 

 ozone has disappeared from tlie air ; and that the decline of the epi- 

 demic and its disappearance is marked by the reappearance of ozone. 

 In rural districts it is nearly always present, while in the interior of 

 large cities it is almost as uniformly absent, owing to the fact that it 

 has been consumed in destroying tlie foul emanations that prevail in 

 these localities. Since ozone is very irritating to the respiratory 

 organs, it has been suggested that the sudden appearance of diseases 

 of the air passages is in all probability due to the occurrence of a 

 wind highly charged with this gas. It certainly is a j^lausible expla- 

 nation of the manner in which influenza will in a single night attack 

 half the inhabitants of a locality. 



It is a well ascertained and indubitable fact that under the 

 influence of sunlight green plants decompose carbonic acid, and set 

 oxygen gas free. This is generally regarded as a direct action of the 

 light, but experiments have shown that if the gas dissolved in water 

 is carefully removed before the plant is introduced, even though car- 

 bonic acid is supplied in suflicient quantity, the plant cannot decom- 



