PHILOSOPHICAL SOCIETY OF "WASHINGTON. 37 



1853. This is also true of the binocular dissecting microscopes 

 made of late years by Beck, of London, while the highly lauded 

 erecting binocular microscope of Mr. J. W. Stephenson, F. R. M. 

 S., (1870-72,) is, in its optical parts, a copy of the binocular com- 

 pound microscope exhibited by Ricfdell at the Cleveland meeting. 

 The latter instrument, as then exhibited, although optically efficient, 

 was roughly put together by Riddell's own hands. The instru- 

 ment exhibited by Mr. Woodward was ordered by Rid dell of the 

 Grunow Brothers, in August, 1853, and delivered to him by them 

 in March following. In its optical parts it is a copy of the model 

 exhibited at the Cleveland meeting, but some improvements were 

 made in the mechanical details of its construction. 



J. S. Billings then made some remarks upon 



THE SCIENTIFIC WORK CARRIED ON UNDER THE DIRECTION OF 

 THE NATIONAL BOARD OF HEALTH. 



Prof. Ira Remsen, of the Johns Hopkins University, has made 

 for the Board an investigation on the organic matter in the air. 

 By the use of tubes, filled with prepared pumice stone, all the 

 nitrogenous matter in the air to be examined, was removed, and its 

 quantity determined by the usual tests for free and albuminoid am- 

 monia. 



Air contaminated by being drawn through water containing de- 

 caying meat does not yield more than the usual quantity of albumi- 

 noid ammonia. 



Air contaminated by being drawn over comparatively dry de- 

 caying organic matter yields more than the usual quantity of albur 

 minoid ammonia. 



Air contaminated by respiration yields more than the usual quan- 

 tity of albuminoid ammonia. 



The simple statement of fact that a given sample of air yields an 

 abnormally large quantity of albuminoid ammonia is not sufficient 

 to enable us to draw a conclusion with reference to the purity of the 

 air. We must know at what season of the year the air was col- 

 lected, and whether in the city or country ; in fact, we should know 

 everything possible concerning the air, and then let the conclusion 

 finally drawn be a resultant of all the facts. It is probable, how- 

 ever, from what is now known, that the determination of the amount 

 of albuminoid ammonia yielded by air may, under many circum- 



