44:2 PROFESSOR W. DITTMAR ON THE BEHAVIOUR OF THE 



a residue containing caustic alkali. This effect must be presumed d priori to be owing to 

 two causes, namely, firstly to the dissociating tendency of the heat, and secondly to the 

 reducing action of the hydrogen on the C0 2 of the carbonate (Ex. Li 2 C0 3 4- H 2 = Li 2 + 

 CO + H>0). Hence, to form an estimate of the share of the heat in the change, a second 

 series of experiments was made, in which the salt was heated in an atmosphere of nitrogen. 

 When I tried this for the first time with carbonate of soda, I was surprised to obtain 

 a residue which contained its caustic alkali chiefly, if not entirely, as NaOH. Where did 

 the hydrogen of the NaOH come from ? Graham's famous experiments on the occlusion 

 of gases by hot platinum afford an easy answer. The hydrogen came out of the flame 

 and through the walls of the red-hot crucible. The effect of this adventitious hydrogen 

 could not have been minimised by establishing ever so rapid a current of nitrogen, 

 because the substance in the crucible is in a state of fusion, and the hydrogen comes to it 

 chiefly direct through the walls, and not by way of the atmosphere, of the crucible. Hence 

 in the subsequent nitrogen-experiments the modus operandi was brought into the 

 following form : — The carbonate was placed in a platinum boat, which was then shoved 

 into a porcelain tube and within it heated in a muffle, matters being arranged so that 

 the boat was at the hottest point of the muffle. The muffle was heated by means of gas 

 in a Fletcher furnace. 



Carbonate of Baryta. 



1. Hydrogen-Experiments. — Carbonate of baryta, prepared from pure chloride by 

 precipitation with carbonate of ammonia, and dehydrated exhaustively at a dull red heat, 

 was placed in the gas crucible, and in it heated ; for the first two hours by means of a 

 Bunsen, then over a gas-blowpipe. The weights of the successive residues was as 

 follows : — ■ 



Original Substance, .... 3427 



After three hours' heating, 

 After four „ „ 



After five „ „ 



After six „ „ 



After seven 



2-7952 



2-667 



2-665 



2-671 (weighing error ?) 



2-667 



0"9592 grm. of the product, by decomposition with dilute hydrochloric acid, gave 

 1 mgrm. of C0 2 , or 0404 per cent, (direct method) ; the solution yielded 1*4462 grm. 

 of BaS0 4 corresponding to 0'9498 of BaO or 99'021 per cent, of the weight of the sub- 

 stance analysed. The deficit of 0"875 per cent, must be water, which, however, was 

 probably taken up between the last heating and the weighing out of the sample for 

 analysis, because hydrate of baryta, as I have shown, is reduced to BaO at a red heat. 

 The product was practically free of carbonic acid, and this is the main point. 



Nitrogen-Experiment. — 2"2168 grm. of carbonate were heated in a current of nitrogen 

 in a porcelain tube within a muffle as above explained, the heating, after attainment of 

 the highest temperature, being continued for one hour. The product weighed only 

 1 # 9 mgrm. less than the original carbonate, which is sufficient to show that carbonate of 



