608 PRINCIPLES OF CHEMISTRY 



calcium (the carbonate, phosphate, borate, and oxalate) are insoluble 

 in water ; besides which the sulphate is only sparingly soluble. As 

 a more energetic base than magnesia, lime forms salts, CaX 2 , which 

 are distinguished by their stability in comparison with the salts 

 MgX 2 ; neither does lime so easily form basic and double salts as 

 magnesia. 



Anhydrous lime does not absorb dry carbonic anhydride at the 

 ordinary temperature. This was already known by Scheele, and Prof. 

 Schuliachenko showed that there is no absorption even at 360 It 

 only proceeds at a red heat, 42 and then only leads to the formation 

 of a mixture of calcium oxide and carbonate (Rose). But if the 

 lime be slaked or dissolved, the absorption of carbonic anhydride 

 proceeds rapidly and completely. These phenomena are connected 

 with the dissociation of calcium carbonate, studied by Debray (1867) 

 under the influence of the conceptions of dissociation introduced 

 into science by Henri Saint-Claire Deville. Just as there is no 

 vapour tension for non-volatile substances, so there is no dissociation 

 tension of carbonic anhydride for calcium carbonate at the ordinary 



42 The act of heating brings the substance into that state of internal motion which is 

 required for reaction. It should be considered that by the act of heating not only is the 

 bond between the parts, or cohesion of the molecules, altered (generally diminished), 

 not only is the motion or store of energy of the whole molecule increased, but also that in 

 all probability the motion of the atoms themselves in molecules undergoes a change. The 

 same kind of change is accomplished by the act of solution, or of combination in general, 

 judging from the fact that a dissolved or combined substance for instance, Kme with 

 water reacts on carbonic anhydride as it does under the action of heat. For the 

 comprehension of chemical phenomena it is exceedingly useful to recognise clearly this 

 parallelism. Eose's observation on the formation (by the slow diffusion of solutions of 

 calcium chloride and sodium carbonate) of aragonite from dilute, and of calc spar from 

 strong, solutions is easily understood from this point of view. As aragonite is always 

 formed from hot solutions, it appears that dilution with water acts like heat. The following 

 experiment of Kiihlmann is particularly instructive in this sense. Anhydrous (perfectly 

 dry) barium oxide does not react with monohydrated sulphuric acid, H2S04 (containing 

 neither free water nor anhydride, S0 3 ). But if either an incandescent object or a moist 

 substance is brought into contact with the mixture a violent reaction immediately begins 

 (it is essentially the same as combustion), and the whole mass reacts. 



The influence of solution on the process of reaction is instructively illustrated by the 

 following experiment. Lime, or barium oxide, is placed in a flask or retort having aa 

 upper orifice and connected with a tube immersed in mercury. A funnel furnished with 

 a stopcock and filled with water is fixed into the upper orifice of the retort, which is then 

 filled with dry carbonic anhydride. There is no absorption. When a constant tem- 

 perature is arrived at, the unslaked oxide is made to absorb all the carbonic anhydride 

 by carefully admitting water. A vacuum is formed, as is seen by the mercury rising 

 in the neck of the retort. With water the absorption 'goes on to the end, whilst under 

 the action of heat there remains the dissociating tension of the carbonic anhydride. 

 Furthermore, we here see that, with a certain resemblance, there is also a distinction, 

 depending on the fact that at low temperatures calcium carbonate does not dissociate ; 

 this determines the complete absorption of the carbonic anhydride in the aqueous 

 solution. 



