272 Recent Progress in Sanitary Science. 



finding as a minimum for the air at Heidelberg 20-84 per cent., 

 and for the maximum 20-97, or a mean for all analyses, of 20-- 

 924. Since that time a great number of inquiries have been 

 set on foot concerning the air in various parts of Europe, 

 especially in Great Britain, where Dr. Angus Smith has in- 

 stituted a very extended series of comparisons between the 

 atmosphere of towns, and of country and mountain districts. 

 The significance of the results is to be found in the fact, that 

 while a falling off in the percentage of oxygen to the amount 

 of one-tenth per cent., may appear so slight as to be un- 

 worthy of serious consideration, yetdhe place of this minus 

 quantity is occupied by other gases whose presence is dele- 

 terious, even when in amounts represented by the hundredths 

 of one per cent. 



Until a very recent period, no similar investigation had 

 been made, so far as we are aware, into the constitution of 

 the atmosphere in the United States. 



It would be fortunate for the interests of sanitary science, 

 if Ozonometry was settled upon as well ascertained princi- 

 ples as those - of the determination of oxygen. But this is 

 far from being the case. The difficulty does not consist in a 

 lack of knowledge concerning the properties, or even the 

 chemical nature of ozone, — to both of which topics a great 

 deal of attention has been paid since the time of Schonbein 

 by Becquerel, Fremy, Andrews and Tait, Meissner, Angus 

 Smith, and others, and in this country by M. Carey Lea, 

 Wetherill, and Rogers, — but to a lack of concerted and sys- 

 tematic observation by practised observers, using equal pre- 

 cautions and pursuing the same methods. To illustrate the 

 discrepancies, and even fallacies which arise, we may instance 

 the ordinary ozone test, as it is called — a strip of paper pre- 

 viously moistened with a mixture of starch-water and iodide 

 of potassium solution, and dried. Recently it was found, on 

 preparing some of the ozone test, that every variety of paper 

 purchasable, except the purest Swedish filter-paper, mani- 

 fested an alkaline reaction to alizarine ; that every sample of 



