> 
1817.] Separation of Silica and Oxide of Tantalum. 107 
may be neglected altogether. A great excess, however, dissolves a 
little, though the portion be still small; and this was the cause of 
the phenomenon which occurred in my analysis, From this we may 
judge to what mistakes we are liable when we employ salammoniac 
to precipitate alumina from caustic potash. 
= 
One of your correspondents has asked the method of separating 
silica from oxide of tantalum. The separation of these two bodies 
is much more difficult than would be believed at first. The oxide 
of tantalum treated by an alkali is soluble in acids, just as happens 
to the silica; while a portion of the silica, on the other hand, remains 
undissolved in combination with the oxide of tantalum. You will 
see in my analyses of the yttro-tantalites how soluble the oxide of 
tantalum is in acids. The only method by which I conceive these 
two substances can be separated is the following :—Fuse the oxide 
of tantalum with bisulphate of potash. Wash off the soluble portion 
of this mixture by means of boiling water. Then dissolve the oxide 
of tantalum by means of quadroxalate of potash, which must be 
boiled for a considerable time over it. The silica remains undis- 
solved, though still retaining a little oxide of tantalum. 
——L 
I shall here make a small addition to the analyses of organic 
bodies which I communicated to you some years ago ; though I have 
not been able hitherto to prosecute the subject much further. 
Formic acid, neutralized by the oxide of lead, gives a formiate 
composed of 
Forinic acid’..4 ie... 25°12 1. da, 1200 
Onide of lead). 0... 6. 79°88 0s 2981 
Hence the capacity of that acid for saturation is 21°314. Ana- 
lysed in the same way as the other acids were, 100 parts of it were 
found to consist of 
ONC eit dance fue am lewaliitinnnn. COL 
LP EE aE a al mS ae ys) 
SI is ins a 90) diel es tieieits “cst aman ten Oe 
These numbers correspond very nearly with the formula 2H + 
2C + 30, which supposes the composition as follows :— 
Lo RN i eg ea 54 arte 2°84 
MOU ate ns os saa ce bec tadeea ls. Cae 
SURUSE ss xe ens hts bois wa ae fice? On. oO 
There is in my result a small excess of carbon amounting to about 
half a per cent. I propose to repeat this experiment as soon as | 
have leisure to recommence my investigations on the composition 
of organic bodies. The comparison of the composition of the 
following acids is curious enough :— 
