8 



Transactions of the Society. 



tated, as clouds, upon the beam, to the action of which they owe 

 their existence. 



The simple apparatus employed in these experiments will be at 

 once understood by reference to illustration. SS' is a glass experi- 

 mental tube, which may vary from 1 to 5 feet, with a diameter of 

 2 or 3 inches. From the end the pipe 'pP passes to an air-pump. 

 Connected with the other end is the flask F, containing the liquid 

 whose vapour is to be examined. Then follows a U-tube T', filled 

 with fragments of marble wetted with caustic potash. The carbonic 

 acid of the air is here removed. Finally comes a narrow tube tt\ 

 containing a tolerably tightly-fitting plug of cotton-wool. This. 



intercepts the floating matter of the air. To save the air-pump 

 gauge from the attack of such vapours as act upon mercury, 

 as also to facilitate observation, a separate barometer-tube is 

 employed. 



The experimental tube SS' being exhausted, a cock at the end 

 S' is carefully turned on. The air passes slowly through the cotton- 

 wool, the caustic potash and the sulphuric acid in succession. Thus 

 purified, it enters the flask F, and bubbles through the Jiquid. 

 Charged with vapour it finally enters the experimental tube, where 

 it is subjected to examination. The lamp L, placed at the end of 

 the experimental tube, furnished the necessary beam. 



We will now permit the electric beam to play upon the invisible 

 vapour of nitrite of amyl. The lens of the lamp is so situated as 

 to render the beam convergent, the focus falling near the middle of 



