VOL. LXVI.] PHILOSOPHICAL TRANSACTIONS. SQ 



a certain quantity of iron filings (v. g. 3J) wrapt up in a bit of paper to prevent 

 its being immediately corroded. This done, he screws the gla'^s vessel to the 

 brass tube, so that no air can get out. When the red fumes begin to rise, he 

 opens the 1 cocks of the brass tube, which open the communication between 

 the glass vessel and the elastic gum bottle. By squeezing the elastic gum bottle, 

 he forces the two airs to mix together. The diminution of the air is soon per- 

 ceived by the elastic gum bottle becoming flaccid. When he judges the air 

 is as much diminished as it can be, he puts the extremity of the glass tube into 

 a vessel with water, and opens the cock of the side tube : the water immediately 

 rises in the glass tube to a height proportioned to the diminution of the two 

 airs. By repeating several times the experiment in the same place, he found 

 the rise of the water nearly the same, though not so exactly as he could have 

 wished: the variation he ascribed partly to the elastic bottle not being always of 

 the same firmness or elasticity, which it loses more or less by squeezing. 



Dr. I. contrived another method, more simple, and perhaps more accurate, 

 which is the following : he took a glass tube about 1^ feet long, and not quite 

 a 12th of an inch in diameter; so that a column of quicksilver might slide 

 through the whole without dispersing itself, filling always the whole cavity. 

 He cemented to each extremity a brass ring, that he might be able to shut the 

 opening with his finger without hurting himself. This tube being divided into 

 100 equal parts, he used it in two different ways; viz. having poured some 

 aquafortis into a little phial, and put to it some filings, he thrust the extremity 

 of the glass tube into the neck of the phial. A column of quicksilver of about 

 an inch in length occupied that end of the glass tube which was in the neck of 

 the phial. The whole was kept in such a posture that the tube was nearly in an 

 horizontal line, the end which is put into the phial being rather the highest. 

 Care was taken that the tube should not touch the aquafortis. The phial being 

 filled with red fumes, and the extremity of the tube surrounded with them, he 

 opens and shuts alternately the opposite extremity of the tubes, so as to allow 

 the quicksilver to advance slowly towards the middle ; as soon as the column of 

 quicksilver is arrived at the middle, he takes the tube out of the bottle, and 

 shuts each extremity with the fore-finger : thus moving the tube upwards and 

 downwards as briskly as can be done with a certainty of keeping both extremities 

 all the while exactly shut. The two airs being thoroughly mixed, he puts one 

 extremity into a vessel filled with quicksilver, and withdrawing the finger from 

 the opening, the quicksilver rises immediately within the tube, and shows by its 

 height the exact quantity of air diminished. 



The other method is this : he ties to the end of the same tube the neck of a 

 small elastic gum bottle, the bottom of which is cut away : having put some 

 iron filings into a little phial, filled with aquafortis, he puts the end of the tube 



