APPENDIX. 679 



When we operate on a substance that boils at 212 or at a 

 higher temperature, its introduction into the balloon is attended 

 with no difficulty. But if it be very volatile, as soon as it comes 

 into the balloon it gives out much vapour, stops the process, or 

 even drives out again the portion which has entered. To reme- 

 dy this the balloon is sprinkled with ether, and we blow upon it, 

 with a bellows to hasten the evaporation. This cools the balloon, 

 and allows the process to proceed ; on the other hand, when we 

 operate upon a substance whose melting point is a little elevated, 

 it becomes solid in the capillary tube and stops the process. To 

 remedy this we take up the balloon with a pair of pincers, and 

 hold it over a charcoal fire, so that the temperature of the capil- 

 lary portion is heated. If we now plunge the capillary point in- 

 to the substance it passes in without becoming solid. 



The balloon being thus charged it is put into the bath in which 

 the experiment is to be conducted. If the matter boils below 

 212, the bath consists of water ; if below 393 we employ a bath 

 of fixed oil ; if above 400, the bath must consist of fusible metal. 

 We might raise an oil bath to 572 or even to 600 ; but we 

 would run the risk of setting fire to the apartment. The expe- 

 riment on that account would require to be performed out of 

 doors. The bath should be such that it can be raised 100 

 above the boiling point of the liquid, the density of whose va- 

 pour we wish to determine. If attention is not paid to this the 

 specific gravity of the vapour will be too high. 



The liquid employed for the bath is put into a cast iron pot. 

 The balloon is attached to an iron triangle, which is kept plunged 

 into the liquid by three leaden weights attached to the ends of 

 the triangle a thermometer is plunged into the bath to indicate 

 the temperature. The fire is lighted, and continued till the bath 

 reaches the boiling point of the liquid in the balloon. Vapour 

 then issues from the capillary beak, and continues till the whole 

 is driven off, and nothing remains but vapour, with which the 

 balloon is filled. We must continue the heat for some time after 

 the evolution of vapour is at an end. The capillary end of the 

 balloon is then hermetically sealed. To see whether the sealing 

 is complete we have only to blow cold air on the beak of the bal- 

 loon. The vapour condenses in the capillary tube into a liquid, 

 but this does not happen if the sealing be not complete. 



No farther precautions are necessary if the bath be water. 



