250 Wisconsin Academy of Sciences, Arts and Letters. 
her as is used by dentists. Its ash, sulphur and chlorine con¬ 
tent have already been given above, and it has also been stated 
that the material was extracted with hot pyridine before using 
it. The rubber so prepared was carefully tied over the orifice 
at E. In this process it was stretched only moderately so as 
not to thin it unduly. 1 In tying the membrane on, the rubber 
was always stretched to such an extent, however, that no folds 
whatever remained where it touched the glass. With proper 
care it is possible to get a smooth surface of rubber to rest 
snugly against the surface of the glass. The rubber was finally 
securely fastened into place by carefully winding the whole 
surface from H to E smoothly and closely with moderately fine 
thread. When these precautions are observed one hardly ever 
fails to get a perfectly tight joint between the glass and the 
rubber, which is so all essential. 
The next step consisted of supporting the membrane so that 
it would withstand pressure. To accomplish this there was 
first tied over the membrane a piece of smooth, soft yet strong 
muslin. This cloth was drawn tightly over the membrane, and 
securely tied on by winding moderately line yet strong thread 
over it smoothly and evenly from H to E. It is of course im¬ 
possible to get rid of folds in the cloth where it is tied on the 
surface H to E; but it is quite possible to distribute these folds 
fairly evenly around the circumference, in such a manner that 
the cloth where it actually touches the rubber on the mouth of 
the thistle tube and just at the edge at E, lies perfectly smoothly 
and snugly against the surface of the membrane giving it 
proper support. After this a circular perforated disc of stout 
sheet steel made of proper size so as to just cover the lower end 
of the thistle tube was placed on the muslin covering the rub¬ 
ber; and this disc was then securely held in place by tying 
over it very firmly another piece of strong muslin by means 
of stout thread evenly and tightly wound on as in the previous 
cases. Thus there was the rubber tied on the glass, then the 
muslin snugly covering and supporting the rubber, then the 
steel disc pressing against and supporting the muslin, and fi- 
iWhen finally in place, the thickness of the membrane was only a 
small fraction of a millimeter. 
