MINERALS CONTAINING BOTH POTASH AND SODA. 
169 
the black and red spaces as distinctly as when 
at rest, except that tlie spaces are curved, 
and, under certain circumstances, increased 
in number. 
If a white disk be employed, with a single 
black space passing from the centre to the 
circumference, and occupying about 20*^ of 
the latter, the effect will not be as in the case 
of the disk of the wheel with only one spoke 
giving the ai)pearance of a complete wheel, 
as in Dr. Roget’s experiment, but the black 
space will be brought out in a curved form, 
and sometimes divided into two. 
If a disk, composed of two semi-circles, one 
white and the other black, be viewed, while 
in motion, from behind the revolving slitted 
disk, the diameter of the disk will vibrate on 
both sides, the centre being fixed; the white 
gaining upon the black and the black, upon 
the white, and so on, alternately. 
The cause, then, of the appearances detailed 
in the first part of this paper, is the same as in 
Dr. Wheatstone’s experiment, the light comes 
and goes before the disk has time to move 
through any sensible space; but, in the ex- 
periments where the light of a lamp flashes 
upon , the painted disk through the slitted disk, 
or where the eye is placed behind the slitted 
disk, the duration of the light is greater 
than the electric light, or than that from 
phosphuretted hydrogen, &c., and the disk 
does pass through a sensible space. Now, 
as the circumference of the disk moves 
quicker than the centre, that is, the velocity 
decreases from the circumference to the 
centre, a black space, for example, seen 
at one point of the circumference, will 
have moved through several degrees as the slit 
pases the eye ; while, at or near the centre, 
the space gone through is barely appreciable. 
This, together with the persistance of impres- 
sions on the retina, added to that which is said 
above, will, 1 think, account for the revival 
of the radii, as also for their curvature ; and 
the rapid succession of black and red spaces 
will account for the apparent increase in their 
number. 
If the distance between the two disks be 
considerable, fourteen or fifteen feet, for 
instance, the curvature of the radii will be 
corrected, and their number will not be aug- 
mented; because, a full view of the diskis 
thus obtained, and the relative velocities of the 
centre and circumference compensated by an 
impression of the whole of the disk being 
formed upon the retina. 
Salisbury i l8th November, 1835. 
ON THE METHOD OF DETERMIN- 
ING THE PROPORTIONS OF PO- 
TASH AND SODA, WHEN THE 
TWO ALKALIES ARE MIXED 
TOGETHER. 
By Thomas Thomson, M. D., F. R. S. 
L. AND E., &c. Regius. 
Professor of Chemistry in the University of 
Glasgow. 
It is no uncommon thing to meet with 
minerals which contain both potash and soda as 
constituents. This is the case, for example, 
with glassy felspar, couzeranite, &c. The 
method of separating the two alkalies from 
each other, in such cases, is that first pointed 
out by Dr. Wollaston. All the other consti- 
tuents of the mineral being seperated, the 
potash and soda are united to muriatic acid, or 
converted into chlorides of potassium and 
sodium. These chlorides being dissolved in 
water, are mixed with a solution ofchloridof 
platinum. The mixture is evaporated to dry- 
ness in a gentle heat, and then digested in a 
sufficient quantity of weak alcohol. The 
chloride of sodium, and any excess of chlo- 
ride of platinum that may have been added are 
dissolved, while the potassium-chloride of pla- 
tinum remains undissolved. Separate it by 
the filter, wash it and dry it; the potash con- 
tained in the mineral amounts to ^ihs, or 
0‘23ofthe weight of this double salt. The 
weight of the potash being knovvn, and like- 
wise the weight of the two chlorides of potas- 
sium and sodium, it is easy to deduce that of 
the soda. 
I consider the following method easier than 
this, especially when the quantity of potash 
and soda to be separated is considerable, and 
I have found that young analysts learn very 
soon to employ it with accuracy. 
1. Convert the mixture of potash and soda 
into sulphates, render these sulphates anhy- 
drous by ignition in a platinum crucible, and 
determine their weight. Let it amount to 
29 grains. 
2. Dissolve the two sulphates in water, and 
throw down the sulphuiic acid by chloride of 
barium. Wash the sulphate of barytes obtain- 
ed, dry it and weight it after ignition. Let 
the weight be 43*5 grains, indicating 15 grains 
of sulphuric acid. 
3. Separate any excess of barytes that may 
have been added to the liquid by the cautious 
addition of dilute sulphuric acid. Filter, 
evaporate to dryness and ignite. The salt 
thus obtained will consist of the mixture of 
potash and soda converted into chloride of 
potassium and sodium. Weigh this salt. 
Let the weight be 24-5 grains. 
Now, the atom of potash is 6, and that of 
soda 4: and it is obvious from paragraphs 1 
and 2 that the mixture of potash and soda 
weighs 14. 
Let the atoms of potash in the mixture be 
X, and those of soda y, it is plain that we have 
U-4y 
6x -j- 4 y= 14 and x= 
By comparing paragraphs 3 and 4, it is 
obvious, that the weight of chlorine in the 
24’5 grains of the mixed chloride obtained is 
13’5 grains. For it must be equivalent to the 
15 grains of sulphuric acid. In this mixed 
chloride the potash is converted into potas- 
sium, and consequently its atom weighs only 
5, while the atom of sodium weighs 3. We 
have, therefore 
ll-3y 
5 X 4 - 3t/ 4 - 13* = 24-5 and x 
5 
If we equate these two values of x we have 
14— 4r/ = 11 —Sy 
6 5 
