18 COHESION AND EXPANSIVE FORCE. 



4. Cohesion and Expansive Force. The particles of 

 a solid can be separated only by force. They are 

 maintained in their natural position by the force of 

 Cohesion. But in gaseous bodies not only is no force 

 required to separate their particles, but, on the contrary, 

 it is only by force that their separation can be prevented. 

 This property, the Expansive Force of gases, is indicated 

 by the expansion of smoke in air, but it is more distinctly 

 obvervable when gases are allowed to enter a vacuous 

 space, that is, a space from which the air has been re- 

 moved. Many gases are not colourless and invisible 

 like air, but are coloured and hence visible ; they are 

 therefore well adapted for these experiments. But 

 these gases are mostly poisonous and nofc easily prepared ; 

 those not well acquainted with the chemical manage- 

 ment of such gases should therefore use smoke for the 

 present purpose. 



Two small glass flasks of equal size are provided with 

 well-fitting corks ; a short glass tube 

 passes through each cork. The project- 

 ing ends of the tubes are connected by a 

 piece of india-rubber tubing, which can 

 be closed by a pinch-cock, fig. 21. To 

 remove the air from one of the flasks a 

 layer of water, G or 8 mm high, is intro- 

 duced into it, and the flask tightly closed 

 by its cork, which remains connected 

 with the cork of the second flask, this 

 being in the meantime placed aside. 

 The pinch-cock being opened, the water 

 in the flask is boiled over the spirit-lamp 

 until a strong jet of steam, issues from 



