96 OPTICAL PROJECTION 



diameter with a roll of woollen material. This is closed by 

 gas-tight caps at each end, with proper nozzles. Ether is 

 poured in to saturate the woollen, and the oxygen passes 

 through the roll. The usual form of this generator sold in 

 England is shown in fig. 56, the tube being double, each 

 length with its own roll of saturated material, the two being 

 connected by a U piece of brass tube. But in his own last 

 construction Mr. Ives prefers a single tube about 14 inches 

 long, which is laid flat, and has a nozzle with stop-cock pro- 

 jecting up at each end. He also now prefers to use petroleum 

 ether, which in the United States is a more common com- 

 mercial product than in England, and volatilises at a lower 



Pro. 56. Porous Generator 



temperature. It also contains no alcohol or water, some 

 small quantity of which is always found in common ether. 

 That is an advantage, but the lower vapour-point I regard as 

 so far a drawback, sulphuric ether itself giving quite sufficient 

 trouble in this respect to make its storage and carriage a 

 matter of some anxiety. 1 



Experiments in England snow that the best saturation 

 is given by the tank, tank vapour requiring decidedly more 



1 Since writing the above pages fundamental changes have been made 

 in the construction of ether saturators; space here forbids further men- 

 tion, but fuller details will be found in the appendix at the end of the 

 book. R. S. W, 



