142 Intelligence and Miscellaneous Articles. 



quartz plates 045 cm. thick. My heat-measuring apparatus was 

 used to determine the transmission of the rajs from a naked gas- 

 jet through this cell. The mean of ten galvanometer readings 

 was 201 divisions. 



The cell was then filled with pure water and the mean of ten 

 readings found to be 196 divisions, showing, if anything, that 

 water is a slightly better absorber than alum solution. When 

 two hundred ohms resistance was added in the galvanometer 

 circuit, the deflexion produced by the unobstructed gas-jet was 

 240 div. Through the water-cell it was 21-5 div. and the per- 

 cent, transmitted was 8*9. When a sheet of plate glass 0*7 cm. 

 thick was interposed the deflexion was 86 div. and the per- 

 cent, transmitted was 35*8. Melloni shows that alum in its 

 crystalline form cuts off all save nine per cent, of the rays from 

 a Locatelli lamp. 



It is not easy to find perfect crystals of alum, but among a 

 large number of ordinary crystals one was found from which a 

 nearly flawless plate was cut. The plate was given a perfect 

 polish on both sides by Brashear's method of working rock-salt 

 surfaces. The plate was 0*37 cm, thick, and. the deflexion of the 

 galvanometer by the gas-jet through the plate was 23*4 div. and 

 the percent, transmitted 9-7. 



Hence it appears that a solution of alum is no better absorber 

 than is water; and furthermore, that it would hardly pay to use 

 plates of transparent alum, even if they were to be had. — American 

 Journal of Science, June 1892. 



ON THE MECHANICAL EQUIVALENT OF HEAT. 

 BY M. C. MICULESCU. 



The author has published in extenso in the Annates de Chimie et 

 de Physique the complete research on this subject, and an abstract 

 in the Journal de Physique. The conclusions at which he arrives 

 are thus stated. 



In this research I have been able to perfect Joule's method by 

 measuring directly the work expended and the heat produced, both 

 measurements having been made by a zero method. 



Moreover, by taking a relatively considerable power (one-horse 

 power) I have increased the quantity of heat produced in unit time, 

 which has enabled me to reduce the duration of the experiment. 



I thus obtained for the mechanical equivalent of heat the 

 numbers 



J = 426-7, 

 in which I consider the units to be quite correct, and the tenths to 

 within 0-7. 



The author remarks that if Joule's observations were reduced to 

 what they would have been at Paris, their mean value would be 



J = 426-5. 

 — Journal de Physique, March 1892. 



