42 VETEEINAEY HYGIENE 



the character of the gas; the most offensive is produced 

 when the diet consists of material rich in nitrogen ; marsh- 

 gas predominates when the diet is largely hay. 



From the decomposition of urea ammonia carbonate is 

 formed, and from its peculiar penetrating odour is the first 

 impurity detected on entering a badly kept stable. 



Of all the impurities we have mentioned there is none so 

 important from a hygienic point of view as carbonic acid. 

 A considerable number of experiments have shown that the 

 amount of this gas present in the air of either rooms or 

 stables, is a direct measure of the degree of purity or im- 

 purity existing, further it is evidence of perfect or imperfect 

 ventilation. 



These results have been established beyond all doubt by 

 the researches of the late Dr. de Chaumont on the air of 

 barrack rooms, prisons and hospitals. He showed that as 

 air impurity increased the carbonic acid increased, and that 

 the amount of carbonic acid present was a measure of the 

 purity of the air. 



It is not the carbonic acid as such which is so important, 

 even in the worst ventilated rooms and stables it rarely 

 exceeds "3 per cent. No toxic effects from carbonic acid 

 are produced unless the gas exists to the extent of five or 

 ten per cent, in the mixture breathed. 



The sole value of carbonic acid determination to the 

 hygienist is the indication which it affords of the presence 

 of organic matter in the air. It has been shown in the 

 experiments above mentioned that these two are intimately 

 related, and that up to a certain point one is a measure of 

 the other. Before this question can be further dealt with, 

 it will be desirable to consider what constitutes the organic 

 matter in the air vitiated by the presence of men and 

 animals. 



Respiratory Organic Matter. — If a quantity of expired air 

 be drawn through distilled water, the liquid has a peculiar 

 odour imparted to it, and rapidly undergoes decomposition. 

 If the air be condensed it can be shown that the organic 

 substance contains nitrogen, can be precipitated by silver 



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