Chemical Science. 181 



quantity of borax. Litmus paper, being rendered red by boracic 

 acid, cannot be used for ascertaining the point of neutrality ; but 

 the tincture of litmus may, because that acid gives it a wine colour 

 only, whilst free sulphuric acid gives it the red colour of onion 

 peel. 



Fifteen grammes of borax were dissolved in fifty cubic centime- 

 ters of water, and rendered of a clear blue colour by a little tincture 

 of litmus. It was then saturated by degrees with sulphuric acid of 

 a known strength from a graduated alkalimetrical apparatus in the 

 ordinary way. The blue solution soon acquired a wine colour, 

 which it retained until the close of the saturation, after which it 

 quickly acquired the red colour due to the excess of sulphuric acid. 

 But to operate with certainty, some precautions are necessary. 



Whilst the solution of borax is hot, the quantity of boracic acid 

 in solution renders the changes of colour less sensible. It is bet- 

 ter, therefore, after having added nine-tenths of the acid necessary, 

 to allow the temperature to fall before the neutralization is com- 

 pleted. As the changes of colour are most easily seen by compara- 

 tive experiments, a little water should be tinted by the litmus, and 

 a drop or two of the test acid added to it, to give it the colour of 

 onion peel ; this liquor is the standard with which the solution 

 of borax is to be compared. As the sulphate of soda and boracic 

 acid in the solution somewhat interfere with the action of the free 

 sulphuric acid on litmus, a mixture of these two substances and 

 water was made in the proportions of the experiment ; tincture of 

 litmus added to it, and then the quantity of acid necessary to give 

 it the standard red colour noted. It required three drops, or a half 

 division on the alkalimeter tube, to produce the effect, and conse- 

 quently that quantity had to be abstracted from the whole quantity 

 used in neutralizing the borax. 



In operating on fifteen grammes of borax, 77.2 divisions of the 

 test acid were required ; this, diminished by three drops or half a 

 division, gave 76.7 divisions, equal in the instrument used to 3.835 

 grammes of concentrated sulphuric acid : according to the analysis 

 by M. Arfwedson, it should be 3.855. 



The process was equally exact in the hands of MM. Welter and 

 Pelouze, who tried it after M. Gay Lussac. It is frequently em- 

 ployed by M. Gay Lussac with advantage in analytical operations. — 

 Annales de Chimie, xl. 399. 



17. Sulphuret of Silicium. Van Mons. — By exposing a mixture 

 of equal parts, of white sand and sulphur, at first to a moderate 

 temperature, and then gradually to one as high as a red heat, a very 

 hard grey porous mass is obtained, insoluble in water and in acids. 

 Acted upon by a solution of caustic potash, it was dissolved, leaving 

 only a few grains of unaltered silica. The liquid, being decanted 

 and left to itself, gradually deposited a fine black powder, which, 



