Intelligence and Miscellaneous Articles. 239 



ration ; its capacity is G litres, and its length GO centime. A vertical 

 cylinder of a litre capacity, communicating at its lower end with 

 the preceding, is immersed inthe same oil- bath ; it contains a piston 

 of the same metal, which by being moved augments or diminishes the 

 volume of the vapour. The rod of this piston passes through a 

 stuffing-box at the toj) of the cylinder; and the vapour can act upon 

 the two faces of the piston. This vapour is supplied from a boiler 

 containing the liquid, and which on the one hand communicates with 

 the horizontal cylinder, and on the other with the upper part of the 

 piston-cylinder. 



When the oil-bath is at the desired temperature, the boiler is 

 raised to the same point ; then the stopcocks are opened, so that the 

 vapour is simultaneously admitted into the two cylinders. The two 

 faces of the piston are then under the same pressure, and it can be 

 moved without any great effort. 



To effect a compression, for instance, communication is cut off 

 between the boiler and the glass-plate cylinder ; a certain quantity 

 of vapour is thus isolated in the apparatus. But the vapour is 

 allowed to act on the upper face of the piston. The piston is then 

 rapidly lowered, and the volume of isolated vapour is thus momenta- 

 rily reduced, which is sufficient for observing the phenomena which 

 take place in this vapour when it is compressed without either ad- 

 dition or subtraction of heat. The construction of the piston offers 

 no difficulty, for it is not necessary that it perfectly retain the vapour. 

 The pressure of the vapour is given by a manometer fitted to the 

 boiler and containing a column of liquid from the mercury to the 

 boiler. By heating the tubes which conduct the vapour into the two 

 cylinders, condensation is prevented and the vapour reaches the 

 glass-plate cylinder dry and saturated. 



The difficulties of construction have been very skilfully surmounted 

 by M. Golaz ; and I have been able to observe the vapours of water, 

 ether, chloroform, and benzine as high as ten atmospheres. The 

 partial condensation of vapour produced by a sudden change of 

 volume is manifested by a mist of very brief duration ; the action of 

 the sides makes it at once disappear. When the mist is very dense, 

 it is seen without difficulty when a mirror reflecting the light of the 

 clouds is looked at through the glass plates ; but when very feeble, 

 it has the appearance of a slight fume which undulates in the cy- 

 linder : the glass plates must in this case be very bright. There is 

 one case in which this is impossible ; and that is, when the vapour is 

 compressed. The action of the sides quickly brings the vapour to 

 its initial temperature, and then a portion of this vapour is necessarily 

 condensed. This condensation produces no mist; but it dims the 

 glasses, and thus observation is rendered difficult. 



This inconvenience has been remedied by looking at the vapour 

 through a telescope applied against the glass, which gives a sharj) 

 image of the central parts of the cylinder; the glass plates are not 

 seen, but the slightest mists which sudden change of volume in the 

 vapour may produce are readily discriminated. 



The following are some of the results of my experiments : — 

 Aquemis vapour is always rendered dull by expansion ; the mist 



