August 17, 1917] 



SCIENCE 



169 



microscopist, versed in physiology, cytology 

 aud liistology. He should be conversant with 

 the chemistry of the coal-tar compounds, not so 

 much from the viewpoint of the maker of dyes 

 as from that of the broad-minded chemist, 

 freed from the econoihic domination of the 

 dye industry, for, as before remarked, fugi- 

 tive dyes, and even colorless compounds, are 

 possible factors in such investigations as are 

 here under discussion. He should have a 

 working knowledge of nemas. 



ILLUMINATION 



In order to distinguish with accuracy among 

 intra-vitam color reactions it is necessary to 

 be very particular about illumination. The 

 most perfectly corrected lenses must be used, 

 both as condenser and objective, and the light 

 used must be as nearly white as possible. The 

 best source of light known to me for these re- 

 searches is bright sunlight reflected from a 

 plane matte white reflector. The reflector 

 should be several feet across, and placed at a 

 distance from the microscope several times its 

 own diameter. It should be universally ad- 

 justable, so that it can be set to reflect a maxi- 

 mum of light to the mirror of the microscope 

 — all the better if heliostatic. A good surface 

 for the screen is made by whitewashing a 

 rather finely woven cotton cloth. 



objective 



Fig. 1. 



The best optical arrangement I have tried is 

 the use of one apochromatic objective as a con- 

 denser for another apochromatic objective. I 

 have been using with success a 2 mm. apochro- 



matic as a condenser for a 2 mm. or 1.5 mm. 

 apochromatic objective. These precautions are 

 necessary if fine color distinctions are to be 

 made with the greatest possible accuracy. If 

 these precautions are taken it will be found 

 that fine distinctions can be made with such 

 precision as to dispel all doubt as to the exist- 

 ence, side by side, in the same cell of definite 

 structures of varying character that it would 

 otherwise be impossible or exceedingly diffi- 

 cult to distinguish from each other. 



Fig. 2. 



The use of an ordinary apochromatic objec- 

 tive as a condenser necessitates the use of a 

 special object slide, consisting essentially of a 

 carrier and two cover glasses. The object is 

 mounted between the cover glasses. Such a 

 slide is shown in the accompanying illustra- 

 tion. The substage of the microscope should 

 have a centering arrangement and a rack and 

 pinion or screw focusing adjustment. A little 

 experience with an apparatus of this sort, in 

 which all known precautions are taken to re- 

 move color from the optical system, leads one 

 to distrust the ordinary Abbe substage conden- 

 ser where fine distinctions are to be made be- 

 tween colors, especially if the colors are of 

 similar character. N. A. Cobb 



TJ. S. Department op Agriculture 



THE AMERICAN CHEMICAL SOCIETY 



DIVISION OP PHYSICAL AND INORGANIC CHEMISTRY 



H. P. Talbot, Chairman 

 E. B. Millard, Secretary 

 The positive and negative specific heat of satu- 

 rated vapors: P. P. Siebel. A vapor expanding 

 from a temperature T to the temperature T — 1 

 reversibly, yields the maximal work W due to the 

 latent heat of vaporization H introduced at the 

 higher temperature in accordance with the second 

 law expressible in equivalent calories as 



_„r-(r-i) 



T 



W = E 



= -— calories. 

 T 



This amount of work is in many eases greater than 



