404 Prof. Tyndall on the Action of Free Molecules on 



" The discrepancy between the two tubes was in no case 

 great, hardly ever exceeding 2 per cent. But the difference 

 was uniformly in favour of the long tube and small density. 

 Diaphragms were employed ; the position of the short tube 

 was shifted ; and it was finally placed so that the pile should 

 occupy the same position in relation to its adjacent end as it 

 did in relation to the adjacent end of the long tube. The 

 discrepancies then disappeared, the absorptions in the two 

 tubes proving practically identical. 



" Many leaves of paper were covered with observations 

 during the week ; but it is useless to take up time and space 

 in copying them here. One representative observation will 

 suffice." 



Sulphuric ether. 



Source of heat — bright-red spiral with rock-salt lens. 

 Long experimental tube. 

 Pressure. Deflection. Absorption. 



linch 20°-0 23-5 



2 inches 27°-3 32'1 



Total heat . 51°-0 



Short experimental tube. 

 Pressure. Deflection. Absorption. 



3-5 inches 17°*8 23«4 



7-0 „ 24°-8 32-6 



Total heat . 49°-0 



The agreement between the two tubes is as perfect as could 

 be desired. 



It is easy to record these experiments; but it is not so easy to 

 make them. On every portion of the apparatus (the source, 

 the tube, the thermopile, and the galvanometer) extraordinary 

 care must be bestowed to make the experiments strictly com- 

 parable. The results were checked by taking the total heat 

 after every experiment — a precaution which ensured the de- 

 tection of any variation on the part of the source. When the 

 platinum spiral was used, the battery of ten Grove's cells, 

 employed to render it incandescent, had to be very carefully 

 prepared, freshly amalgamated zinc plates being used in each 

 fresh battery. The requisite deflection of a tangent compass 

 being produced, it was kept constant by means of a rheochord 

 throughout the day. When the battery showed signs of rapid 

 falling, it was always renewed. It would be tedious to dwell 

 upon the precautions taken to protect the source and the pile 

 from the least agitation of the air. Such precautions are 

 essential; but their necessity and form must be learnt by each 

 experimenter for himself. 



