ASS \Tiyr, ] 



CHEMISTRY. 



407 



to precipitate the whole of the acid as sulphate of baryta. 

 When the precipitate is complete, filter, wash, and dry 

 as before ; and after drying, weigh with the filter. 

 With such a quantity of the sulphate of potash, a pre- 

 cipitate of twenty-nine grains of sulphate of baryta will 

 be produced. Now the equivalent of sulphate of baryta 

 = 1 1(5 ; and that of sulphuric acid in the anhydrous state 

 = 40. And hence we have, 



As 116 : 20 : : 40 : 10 

 29 



SCO 



116) 1100(10 grains. 



1100 



Hence we find that twenty grains of sulphate of potass 

 contain, according to the analysis, ten grains of sul- 

 phuric acid. The actual composition of the salt is 



87-2 



So that a little error has been made in such an analysis. 

 The preceding will afford initiative experiments for 

 the beginner; and by practising simple examples, he 

 will gradually acquire the dexterity and accuracy only 

 obtained by constant practice and experience. 



In quantitative analysis, there are some processes 

 fit- '* 



wliich are specially applicable for certain purposes. One 

 of these, cupellatiou, we shall now describe, because it is 



Fig. 76. 



extensively used both by analytical chemists and by 

 metallurgists in refining and assaying. A peculiar form 

 of furnace is requisite for this operation, and an instru- 

 ment called a muffle. In the preceding engraving, a view 

 of a furnace externally, and as arranged for the reception 

 of a muffle, is shown. On the left hand view, an open- 

 ing is seen in the body of the furnace, into wliich the 

 muffle is introduced. 



The muffle is made of earthen- 

 ware, and in this the cupel is 

 placed. A view of an ordinary 

 form of the muffle is represented 

 in the margin. The cupel itself is 

 a small vessel, made of bone ashes, 

 which are employed on account of their porosity. 



The operation of cupellation depends on the fact, that, 

 when gold and silver, which do not become oxidised at 

 a red heat, are alloyed with metals that do, the latter 

 separate from the noble metals, leaving them in an al- 

 most pure state. Supposing silver is to be assayed, a 

 few grains are put into some pure lead foil. The two 

 are then placed in the muffle, already red-hot in the 

 furnace. By the peculiar arrangement of the whole 

 apparatus, a current of air passes over the silver and 

 lead ; and so long as lead or any other readily oxidisable 

 metal remains, the two present a dull appearance. The 

 operator must carefully watch ; and quite suddenly he 

 will perceive the silver to present a most brilliant ap- 

 pearance, owing at that moment to the almost entire 

 removal of all the baser metals. The process is then 

 concluded ; and the cupel having been allowed to cool, 

 the button of pure metal may be removed and weighed. 

 The cupellation of gold is conducted in a similar manner, 

 with the exception that silver and lead are both em- 

 ployed, instead of lead by itself. By this an alloy of 

 silver and gold is produced at the conclusion of the 

 cupellation. The silver is afterwards removed by means 

 of nitric acid ; the gold being thus left in a pure state. 



This process is one requiring considerable care ; for at 

 times the metals, at the instant of their becoming pure, 

 swell up. This is especially the case with silver ; for 

 its bead occasionally bursts, by which a loss of metal is 

 sustained. 



With this we shall conclude our remarks on quantita- 

 tive analysis, so far as inorganic bodies are concerned. 

 The subject is of so extensive a character, that were we 

 to attempt to give full details, we should have to far 

 exceed the limits which our plan imposes on us. We 

 have already named the best works which the student 

 can consult.* In them almost every substance which 

 can be met with, is treated on in full particulars. 

 Analysis, from its numerous uses, has become a distinct 

 branch of chemical science ; and one in which, to be 

 proficient, requires an undivided attention. Our object 

 has been to give a general idea of the subject, so that 

 the beginner may have an opportunity of learning lead- 

 ing principles and methods only. 



CHAPTER VI. 



THE CHEMISTRY OF VEGETABLE SUBSTANCES, AND THEIR PRODUCTS. 



IT has long been customary to arrange the description of 

 the chemical properties of organic bodies under the head 

 of Organic Chemistry ; but owing to the extension of 

 OUT knowledge during the last few years, the distinction 

 which had existed between the animate and inanimate 

 creation, in a chemical point of view, has been removed, 

 or so greatly modified as to scarcely exist. We propose, 

 therefore, to devote the following chapters to a descrip- 

 tion of such bodies as exist in, or are, the products of 

 vegetable substances only. With respect to the che- 

 mistry of animal life, that has been so fully dilated on 

 in the chapter on Food, <fcc., as to leave nothing to add 

 to it. 



During the last twenty years, the study of the che- 

 mistry of vegetable substances, &c. , has made astonish- 

 ing progress. To Dr. Liebig, of Giessen, we are pre- 

 eminently indebted for masterly and accurate researches 

 in this branch of experimental science. It has been 

 chiefly owing to his discoveries that so deep an interest 

 has been generally taken in the subject, and that agri- 

 fiilture has advanced to the rank of a science, in place of 

 being a pursuit of chance and speculation, as it had 

 formerly existed. 



In other branches of industry, similar results have 

 accrued, owing to the rapid applications which have been 

 See ante, p. 397. 



