526 C. C. Hutchins — Absorption of Radiant Heat. 



Aet. LXVI. — Note on the Absorption of Radiant Heat by 

 Alum ; by C. C. Hutchins, Bowdoin College. 



It seems to be the generally accepted opinion that a solution 

 of alum acts as a particularly efficient absorber of rays of great 

 wave length. It is certain that the alum cell is universally 

 used and recommended wherever a transparent medium is 

 desired for cutting off the so-called " dark " heat rays. 



In the April number of this Journal Mr. F. J. Rogers states 

 that the results of Julius Thomsen are in error because Thom- 

 sen used pure water instead of alum solution as an absorber. 



Whence this idea in regard to the effect of alum solution arose 

 it is difficult to say : I have searched in vain for any authority 

 for it. Melloni's table, as far as it goes, shows that bodies in 

 solution raise slightly the percentage of transmitted rays above 

 that which passes through pure water. 



I prepared a saturated solution of potash alum in distilled 

 water and placed it in a cell 0'6 cm thick. The sides of the cell 

 were quartz plates - 15 cm thick. My heat-measuring apparatus 

 was used to determine the transmission of the rays 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 were added in the galvan- 

 ometer circuit, the deflection 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 - 7 cm thick was interposed the deflection 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 Locatellilamp. 



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 - 37 cm thick, and the deflection 

 of the galvanometer by the gas jet through the plate was 

 23 - 4 div. and the per cent transmitted 9*7. 



Hence it appears that a solution of alum is no better ab- 

 sorber than is water ; and furthermore, that it would hardly 

 pay to use plates of transparent alum, even if they were to be 

 had. 



