828 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[April 19, 1873. 
sulphuric acid ; and, thirdly, I use weighed quantities of 
reagents, and as little as possible of them, and at the same 
time avoid all unnecessary dilution. Attention to these 
particulars renders the detection of alum in bread a 
certainty. I will give an outline of the process. 
The ash from 100 grams of flour weighs 700 milli¬ 
grams, and, in a case of aluminized flour, may contain 
some 30 milligrams of alumina in addition. I moisten 
this ash with a measured 0’5 cubic centimetre of oil of 
vitriol, then heat up until the oil of vitriol 'begins to 
volatilize, whereby the silica is rendered insoluble and the 
attack of the alumina in the ash is insured. Having done 
this, and allowed the ash to cool, I dilute with a little 
water and filter. The filtrate is then treated with 1J gram 
of pure caustic potash, which renders it alkaline and 
redissolves the alumina. The solution is then filtered, 
and the filtrate treated with 1-| gram of chloride of 
ammonium, and boiled and allowed to stand, whereupon 
the alumina is precipitated as phosphate of alumina, which 
admits of being got on a filter, washed, ignited, and 
weighed. 
The advantage gained in this instance by using weighed 
quantities of reagents, and by avoiding dilution, will 
commend itself to chemists, who will not fail to recognize 
that, though there be difficulty in insuring that an indefinite 
quantity of acid and alkali shall be quite free from alu¬ 
mina and silica, there is not much difficulty in getting 
I gram of sulphuric acid, 1J gram of potash or soda, and 
II gram of chloride of ammonium, so as collectively not 
to contain a couple of milligrams of alumina or of silica. 
The testing whether flour be sound or not by the 
strength of the aqueous extract is not new, but has been 
developed and rendered easily practicable in the course of 
my experiments. The basis of the method is, that in 
sound flour there is very little sugar and dextrine, but that 
in unsound flour there is either much sugar and dextrine, 
or else that a short exposure to the action of water 
converts much of the starch of the flour into dextrine and 
sugar. In the space of an hour and a half, I can by a 
little management make a determination of the quantity 
of extractive given by a sample of flour. I take 100 
grams of the flour, mix it well with some water, and then 
■dilute the whole mass with water until it occupies exactly 
half a litre. I then pour it on a dry paper filter in a dry 
funnel, whereupon the liquid runs through tolerably rapidly 
at first ; by and by, as is known, such a filter will become 
clogged up, but not until at least some 50 cubic centi¬ 
metres of filtrate have run through. These may be mea¬ 
sured and evaporated down to dryness in the water-bath, 
and the residue weighed. Ten times the residue yielded 
by the 50 cubic centimetres is equal to the amount of 
-extractive yielded by 100 grams of the flour. The extrac¬ 
tive furnished by 100 grams of sound flour is 4*69 grams. 
Of this, 0'44 gram is ash and 0 - 92 gram vegetable 
albumen, leaving 3*33 grams of dextrine, sugar, and gum. 
If sound flour be left for twenty hours in contact with 
■cold water, and then the aqueous extract be taken, it will 
be found to be increased somewhat : I found it to be 6’01 
grams. In unsound flour, Odling has found 12 and 
18 ‘2 grams of extractive per 100 grams of flour. I believe 
that Odling first proposed the determination of the amount 
of extractive yielded by flour to cold water as a test of the 
soundness of flour. I am inclined to the belief that this 
■test may be made very practicable and valuable. 
THE VAPOUR-DENSITY OE POTASSIUM.* 
BY JAMES DEWAR AND WILLIAM DITTMAR. 
Since the elaborate experiments of Deville and Troost 
on the vapour-densities of substances at high tempera¬ 
tures, little has been added to chemical science in this 
field of research. Doubtless this is in great part owing 
to the difficulty of any one student manipulating the com¬ 
* Paper read before the Royal Society, March 6, 1873. 
plex apparatus necessary for the execution of the experi¬ 
ments. But the operations are greatly increased in diffi¬ 
culty when we select bodies that are readily inflammable 
in air and attack with facility glass and porcelain at the 
high temperatures to which they are exposed. This is 
the reason why the molecular weights of a most important 
class of elementary bodies, viz. the alkali metals (although 
these are volatile at moderate temperatures), have re¬ 
mained to the present time undetermined. It was with 
the view of adding something to our knowledge in this 
department, that we recently undertook some experiments 
with potassium, the results of which we now beg leave to 
lay before the Society. The special difficulties we had to 
overcome are involved in the endeavour to answer the 
following questions :— 
1. Is it possible to convert potassium into a gas of 
one atmosphere’s pressure at any of the constant tempera¬ 
tures we can at present command ? 
2. Is it possible to generate pure potassium-vapour and 
to keep it from getting oxidized ? 
3. Supposing a definite volume of such vapour to have 
been procured, how can its weight be ascertained ? 
After a succession of failures, which we shall not detail, 
we at last succeeded in devising a workable process, which 
may be briefly described as follows :— 
A cylindrical iron bottle of at least 200 cub. centims. 
capacity, of a thickness in the body ensuring sufficient 
rigidity at even a bright red heat, and provided with a 
well-ground inbent neck, pierced with a canal of about 2 
millims. in diameter, is employed as a generator and re¬ 
ceptacle of the vapour. 
A mass of about 20 kilogrs. of zinc contained in a plum¬ 
bago crucible, which, being placed in a forge-fire, can be 
readily heated up to the boiling-point, serves as a bath. 
The experiment begins by first deoxidizing the inside 
of the receptacle at a red heat by means of a current of 
dry hydrogen, which is continuously maintained until the 
bottle has cooled down below redness. At this stage 
about 200 grms. of pure mercury are introduced into the 
bottle, which is then inserted into the red-hot zinc, with¬ 
out, however, covering the upper extremity of the bottle. 
After | of the mercury is distilled off (which is accom¬ 
plished in a very short time), the neck is withdrawn, and 
while the mercury-vapours are still streaming out, an iron 
test-tube, previously prepared with great care and charged 
with 4-5 grms. of potassium, is dropped into the bottle, 
the neck reinserted, and after the whole of the bottle has 
been immersed into the zinc, the blast of the forge is 
forcibly increased so as, in the shortest possible time, to 
bring the zinc into the state of boiling, proper arrange¬ 
ments being made for keeping the neck of the bottle red- 
hot. The potassium in a short time begins to volatilize, 
issuing in jets into the air, and depositing caustic potash 
at the nozzle, which must be kept clear by means of an 
iron wire. As soon as the distillation of the potassium 
ceases, the nozzle is closed by means of a ground-in wire 
plug, at once immersed into a mass of mercury contained 
in a test-tube, and the bottle withdrawn to a proper sup¬ 
port, on which it is allowed to cool. 
After it has reached a manageable temperature, the 
bottle is inserted into a mass of recently boiled water, the 
wire plug withdrawn, and the hydrogen formed by the 
action of the water on the potassium pumped out, by 
means of a “ Sprengel,” into a eudiometer, to be measured. 
In the experiments we have hitherto carried out, we 
have satisfied ourselves that the amount of mercury-vapour 
not swept out by the potassium is quite inappreciable ; 
and as our object has been in the meantime to merely 
arrive at approximate results and to perfect our methods 
of manipulation, we have neglected the minute correction, 
which, on account of that small remnant of mercury, 
ought, strictly speaking, to have been applied to the 
volume of the vapour as calculated from the capacity of 
the bottle in the cold, the coefficient of expansion of iron, 
and the temperature (1040° Deville) at which the vapour 
was measured. 
