HYDRATES AND CARBONATES OF THE ALKALI-METALS, ETC. 447 



The absorption apparatus were tared in two ways, namely, firstly, in the ordinary 

 way by means of weights, and secondly, in the Regnault fashion, i.e., by means of 

 similar apparatus of a slightly less weight, and of very nearly the same displacement. 

 The tare potash-bulbs were charged with plain water, the tare U-tubes with granulated 

 selenite. In the calculations I relied on the results of the Regnault method ; the 

 other, indeed, was used only as a guarantee against weighing-blunders. I may say, in 

 passing, that the two values for the weight of C0 2 found always agreed to within less 

 than 1 mgrm. 



The apparatus having been put together, and the tightness of the joints made sure of, 

 the carbonate was decomposed slowly by addition of dilute sulphuric acid, the Liebig 

 bulbs being kept cool by occasional application of a water-bath ; when the carbonate was 

 fully decomposed, the absorbed part of the gas was expelled by heating the contents of 

 the flask to very near boiling, and passing a current of carbonic-acid-free air through the 

 apparatus at the same time. This was continued until an approximate calculation, based 

 upon the known volume of air used and the volume of the empty space of the apparatus, 

 showed that the carbonic acid was sure to have all been driven into the absorption 

 apparatus. These were then detached, immersed for a time alongside of the tare 

 apparatus in a water-bath of the temperature of the balance, wiped dry, and weighed. 

 Each apparatus and its tare were kept in the balance-case, with the ends closed, until 

 their difference of weight was constant, the caps which closed the ends of the U-tubes 

 and of the potash-bulb being removed in the actual weighing not having been included 

 in the tare. The weighings here, as in most of the analyses quoted in this memoir, were 

 made with a set of iridio-platinum weights from Messrs Johnson, Matthey, & Co., but 

 adjusted by Mr Oertling. When the set came to me, I determined the errors of the 

 several pieces by a series of comparisons and readjusted a few of the pieces which needed 

 it. The balance used was that very fine " Hectogramme-balance " from Oertling, 

 which I refer to in my article " Ueber die Pracisionswaage des Chemikers," in the 

 Zeitschrift fur Instrumentenkunde for 1881, p. 113 et seqq.; the instrument is provided 

 with the microscopic reading arrangement, which I contrived some years ago and described 

 in the same Journal (for 1882, p. 63). 



As already stated, the first set of analyses was made with a carbonate, which, after 

 dehydration in carbonic acid, was fused in an atmosphere of the same gas of about 

 32 inches pressure. As a general result, the percentage of carbonic acid found was higher 

 than that demanded by the formula, if Li = 7 '02 (Stas), and = 16. This confirmed my 

 surmise regarding the true value of the constant, apart from the consideration that my 

 carbonate, having been produced in an atmosphere of carbonic acid, may have contained 

 physically absorbed, in addition to its combined, C0 2 . Assuming this to be so, the 

 weight of C0 2 present per unit-weight of oxide of lithium should be governed by 

 an equation like y = A + Bp, where p stands for the C0 2 -pressure under which the 

 substance was fused and A for the chemically combined part of the carbonic acid. 

 To find the values of the constants A and B, I prepared a few samples of the 



