314- Chemical Society. 



of Aggregation of the Molecules of Bodies," by Warren De la Rue, 

 Esq. 



The subject of this short notice is the practical application of the 

 action which takes place in masses, composed of palpable particles, 

 when raised to a temperature insufficient even for their partial fu- 

 sion. 



In illustration of the particular action alluded to, may be quoted 

 the following familiar facts : — Precipitated gold, when heated to a low 

 red heat, contracts in volume, becomes more coherent and yellow in 

 colour ; clay contracts in volume when heated, and generally in pro- 

 portion to the intensity of the heat ; the carbonaceous deposit in the 

 inside of gas retorts, by the continued action of heat, acquires suffi- 

 cient hardness to scratch glass ; ordinary coke and charcoal become 

 harder the longer the action of heat is continued on them ; these and 

 many other analogous facts are examples of a new molecular arrange- 

 ment being produced in various substances, by subjecting them to an 

 increase of temperature, not however sufficient for their fusion. 



To cause the foregoing changes a red heat is employed ; we shall 

 however presently see that a temperature but little above that of 

 boiling water is quite sufficient to materially alter the cohesion of 

 some substances. 



It may be as well here to premise, that the particles should be 

 brought as closely as possible together ; to effect this, if the sub- 

 stance be in powder, it must be made into a paste with water to 

 displace the air, and the paste so prepared submitted to a pressure of 

 four tons or upwards on the square inch ; air being so exceedingly 

 compressible it cannot be got rid of without the use of some liquid. 

 The manner of pressing need not here be entered on, the operation 

 being purely mechanical. 



White lead precipitated by carbonic acid gas from a hot solution 

 of the sub-nitrate always falls as an exceedingly light deposit ; if it 

 be pressed as before described, and the pressed cake dried at the or- 

 dinary temperature of the atmosphere, it coheres but imperfectly, but 

 on being subjected to a heat of between 200° and 300° Fahrenheit, 

 it becomes exceedingly hard and compact ; and if the cake be ground 

 up with water and redried, it will be found far more dense and opake 

 than the original precipitate, showing the change to be permanent. 



The following fact was communicated to me by Messrs. Nasmyth 

 and Co. of Patricroft : — Common chalk cannot readily be sawn into 

 thin slips, as it crumbles under the operation ; if however it be baked 

 at the temperature before named it becomes far more tenacious, and 

 may be then cut into any form we choose, still being sufficiently soft 

 for drawing or writing, to which purposes it is far more applicable 

 than before baking. 



Almost all precipitates dry much more crisp at high than at low 

 temperatures, the agency of heat facilitating the attraction of such 

 particles as may happen to be in contact. 



In conclusion, I may remark that it appears by no means impro- 

 bable that the long- continued action of temperatures, but slightly 

 elevated above the ordinary temperature of the atmosphere, may 



