Gabbroid Rocks of Mi?mesota. — Winchdl. 373 
account of the lower heat applied^ and on account of the dilu- 
tion of the hydrogen with HCl gas. 
3. It requires much less attention during the reaction; 
with the old method it is necessary to change the currents 
every three or four minutes; with this arrangement once 
started, the apparatus can be left to itself till the reaction is 
complete, provided the hydrogen generator will continue to act 
for that length of time. 
4. This arrangement permits the simultaneous qualitative 
determination of titanic acid. This last point requires a word 
of explanation. Titanic acid (TiO:;) often occurs in the form 
of a white powder which cannot be distinguished from alu- 
mina AI-O3. This powder is aflfected by the Deville-Rivot 
method, but only very slowly, and with uncertain results. 
Ordinarily* by this method the titanic acid is partially changed 
{often only a small part) to sesquioxide of titanium (TiiOs), 
which colors the alinnina a dirty gray. . This is not distinct; 
it is only obtained after long treatment, and not characteristic, 
since traces of iron can produce practically the same color. 
On the other hand, by the method described above the titanic 
acid is changed to the intermediate oxide TiaOs, which has a 
characteristic color, being dark gray or indigo blue, and re- 
veals its presence by the tint imparted to the alumina. With 
10% TiO^ in the powder the color is very marked slate blue. 
However, the test is not very delicate and the results of the 
analyses show that it becomes rather uncertain for less than 
4% of titanic acid in the powder after separation of the iron. 
That is, in case the alumina attains 25% of the rock, this meth- 
od will reveal \% of titanic acid. If the alumina is less, a pro- 
])ortionally smaller per cent of titanic acid can be detected. 
It should be remembered in this connection that some titanic 
acid can usually be found with the silica ; but a large part is 
usually found with the iron and alumina. 
As a check upon the iron determination, the u'on was also 
determined in both its conditions by the Cooke method as 
modified by J. H. Pratt f, with very satisfactory results. 
*But Van der Pfordten obtained TiOs after treatment for several 
hours in dry hydrogen gas at very higli temperature. See Deutsch. 
cliem. Gesellsch, i(S84. I. p. 728. 
tj. H. Pratt: American Journal of Science; 1894: XLVIII. p. 149. 
