186 



Dr. W. Marcet. On the 



should any water accidentally reach the solution it will always 

 remain saturated. This solution answered its requirements in every 

 way. The thickness of the glycerine enabled it to adhere to the 

 fragments of pumice-stone in the form of a thick layer, while the 

 water of the potassic solution was so firmly retained that none 

 was given out to the current of air passing through it. Placed over 

 sulphuric acid under a receiver a sample of this mixture underwent 

 no loss of weight. This fluid absorbs carbonic acid with the greatest 

 speed, and I am not yefc able to say when its power becomes 

 exhausted. It is held in bottles kept near the absorption-tubes, and 

 before each analysis about 200 c.c of the solution is poured 

 into the copper cylinder, when it trickles down the pumice-stone 

 appearing shortly afterwards in the exit tube, when it is let out into 

 the bottle. The bulb blown on the exit tube is indispensable as a means 

 of collecting the solution which remains behind and would otherwise 

 stop the communication between the bell-jars — an accident which 

 frequently happened until the present arrangement was adopted. 

 The air after leaving the absorption apparatus just described passes 

 over pieces of caustic potash, where it deposits any last traces of 

 carbonic acid which may have escaped from the absorbing fluid ; it is 

 finally cooled by the water-jacket, and in this state is collected in the 

 right-hand bell-jar. 



The heat produced by the combination of the carbonic acid with 

 the alkali introduced, was at first a serious difficulty in the working of 

 my instrument. Various means were tried either to get rid of it or 

 estimate its influence so as to correct it by calculation. At last I 

 succeeded — I may say beyond my expectations — in doing away entirely 

 with this difiiculty. The means I employed was surrounding the 

 absorption-tube with the non-conducting material glycerine, and 

 determining the temperature of the air in this tube with a very deli- 

 cate thermometer. 



The measuring apparatus consists of a graduated receiver, holding 

 about 800 c.c, and moving up and down over mercury contained in a 

 strong glass trough. The receiver is drawn out at the top into a long 

 tubular neck, and this is fixed by a bracket to a rack and pinion 

 movement. The opening in the neck is connected by india-rubber 

 tubing and a combination of "|"-pieces, with a gauge on one hand and 

 the bell-jars on the other, while a small tube of calcium chloride is 

 interposed in such a way as to dry the atmospheric air as it is drawn 

 into the graduated glass receiver. 



The gauge is a very pretty instrument, which was made for me at 

 the works for the construction of physical instruments at Geneva. It 

 consists of a glass U _ ^ UDe which can be raised or depressed by a 

 rack and pinion movement, and a web stretched across the tube, though 

 fixed to an independent brass-piece. By this simple arrangement it is 



