Apbil 5, 1907] 



SCIENCE 



527 



the investigations of Clausius and Max- 

 well, can not be correct. 



On the Variation of the Heat of Mixture 

 with Concentration and Temperature: 

 B. M. Clabk. 



"When two liquids are mixed, there is in 

 general either an absorption or a genera- 

 tion of heat, the amount of heat depending 

 upon the relation of the liquids present, 

 and upon the temperature at which mix- 

 ture occurs. 



The variation of the heat of mixture with 

 temperature— for any one definite concen- 

 tration — bears theoretically a simple rela- 

 tion to the specific heats of the components 

 of the mixture and the specific heat of the 

 resultant mixture, and the present experi- 

 ments were begun with a view to determia- 

 ing experimentally the numerical value of 

 this relation in the case of some of the 

 simpler solutions. 



Observations were made on the following 

 liquids : 



Glycerin — Water, 



Glycerin — Ethylalcohol, 



Glycerin — Methylalcohol, 



Phenol — Water, 



Isobutylic Alcohol — Water, 



Anilin — ^Xylol, 



Anilin — Toluol. 



The values obtained for the heat of solu- 

 tion at various temperatures show that for 

 the liquids tested the thermodynamic rela- 

 tion 



dQIdt^K—K', 



holds within the limits of experimental 

 error. The value dQ/Bt known as the 

 temperature coefScient of the heat of mix- 

 tures varies in some mixtures with the 

 temperature. 



Anilin xylol, anilin-toluol gave evidence 

 of a temperature coefScient equal to zero, 

 i. e., 



dQiat = o. 



Numerical values and curves will be 

 given in a paper to appear shortly. 



A Color Mixer: Albert B. Pobtee, 



Chicago. 



The simplest means of mixing and 

 matching colors is undoubtedly Maxwell's 

 color-top. It is, however, defective in that 

 one can not continuously vary the positions 

 of the slotted discs while seeking a color- 

 match, but must usually stop the top many 

 times, readjust the discs, and again set up 

 the rotation, before a satisfactory match is 

 secured. 



In the present instrument the slotted 

 discs remain stationary while their image 

 is rapidly rotated. This is effected by 

 viewing the discs through a right-angled 

 prism which rotated with its hypothenuse 

 parallel to the axis of rotation and to the 

 line of sight. As the slotted discs remain 

 stationary, their angular exposure can be 

 easily and continuously varied until the 

 best possible color-match is secured. 



On the Nature of Optical Images: Albert 



B. Porter, Chicago. 



A consideration of the method by which 

 light is propagated is sufficient to show 

 that an ordinary optical image is merely a 

 particular case of an interference pattern. 

 This may be easily shown experimentally 

 by using as an object a coarse, black-line 

 grating illuminated by a parallel beam of 

 monochromatic light passing through the 

 grating and then through a convex lens. 

 On the far side of the lens a system of 

 sharply defined interference fringes is 

 formed which can be seen with an eye-piece, 

 or, intercepted on a screen, at any point 

 over a considerable range along the axis. 

 Somewhere in this system of fringes is the 

 geometrical image of the grating, but it is 

 visually quite indistinguishable from other 

 sections of the fringe system. If the angle 

 of incidence of the light falling on the 

 grating is changed, the whole fringe sys- 

 tem shifts to one side or the other except in 

 the focal plane, where it remains sta- 



