98 
POPULAR SCIENCE J^EWS. 
[July, 1891. 
acid (H, SOj) . Chlorine is generated, and the cloth 
is bleached. 'J'his method is much better than the 
use of chlorine gas, because it gives only the 
amount of chlorine needed, and only at the place 
where it is needed— in the fibres of the cloth. 
[Pharmaceutical Journal.] 
THE NATURE OF SOLUTION. 
An answer has long been sought to the question 
"What goes on when a solid substance is dissolved 
by a liquid V " Those who received their course of 
didactic instruction a good many years ago were 
taught that the solid substance was liquefied. It 
was said that the particles of a fusible salt, such 
as nitre, for instance, could be brought into the 
liquid condition by one of two methods : tiie first 
that of applying heat, heat-energy being used up 
in the process of liquefaction ; the second process 
being that of submitting the body to the action of 
water, when the absorption of heat due to lique- 
faction was shown by the lowering of tempera- 
ture. This explanation of the process of solution 
has been long since found unsatisfactory, and in 
more recent years teachers have preferred to say 
as little as possible about a process in appearance 
so simple but of which the explanation was 
found to be encompassed with difficulties. Of 
late, however, the question has assumed another 
aspect. We have not attained, it is true, to unani- 
mity of opinion as to what goes on in the process 
of solution, but there is nevertheless a perfectly 
definite theory of the process now current, which 
has the strenuous support of a prominent school 
of continental chemists. A complete account of 
the new theory has recently appeared in an acces- 
sible form in Oswald's 'General Chemistry.' In 
this work the pros rather than the cons of the 
question are dealt with. Exceptions yet remain 
to be explained, and fundamental experiments 
will have to be repeated and confirmed by inde- 
pendent workers, before the new theory can be 
looked upon as firmly established. Whatever 
may be the final judgment of the chemical world, 
the adequacy of the new theory to explain the 
phenomena of solution is undoubtedly one of the 
great scientific questions of the day. 
The following is a short resume of the theory 
and the data on which it is based. 
The particles of a body in solution are not in 
the liquid but the gaseous condition ; that is to 
say, they obey laws having exactly the same form 
as the laws of gases. In diluted solutions the con- 
ditions are similar to those in a perfect gas ; con- 
centrated solutions show deviations from the 
simple laws which are similar and in the same di- 
rection as the deviations from the laws of Boyle 
and Charles shown by gases near the point of 
liquefaction. If pure water be poured on the 
surface of a solution of sugar the particles of 
sugar rise against the action of gravity and mix 
with the pure water. There are certain sub- 
stances with which we can build up a " semi-per- 
meable wall" which will allow the water to pass 
but not the dissolved substance. A porous earth- 
enware cell in and on which is deposited copper 
ferrocyanide forms such a wall. Its properties 
are not the same as those of an animal membrane, 
such as parchment, which will allow a crystalline 
substance in solution to pass through it. If a 
cell containing a sugar solution be provided with 
such a semi-permeable wall and is further con- 
nected with a pressure guagc, then on immersing 
the cell in a vessel of pure water, water passes 
slowly into the cell, the sugar does not pass out, 
and the particles of the dissolved substance exer- 
cise a pressure on the solvent which is registered 
by the manometer. TTie maximum pressure is 
only obtained slowly, but is of xonsiderable mag- 
nitude, a one per cent, solution of sugar showing 
a maximum pressure of about 50 c. m. of mer- 
cury. The arrangement described affords us then 
a means of measuring the pressure exerted on the 
solvent by the particles of a substance in solution. 
It is found that this pressure is directly propor- 
tioned 10 the concentration of the sugar solution. 
All the other substances which have been investi- 
gaten follow the law — pressure varies as concen- 
tration. It will be noticed that this law is of ex- 
actly the same form as that of Boyle, which 
states that the pressure of a gas varies inversely 
as the volume, t". e., directly as the concentration. 
Again the pressure of the dissolved substance 
increases as the temperature rises, and for all sub- 
stances so far investigated the rate of increase is 
the same as in the case of gases. With gases, if 
the volume be maintained constant, the pressure 
varies directly as the temperature reckoned from 
the absolute zero of the air thermometer. This is 
Charles' law, followed, as has been said, by dis- 
solved substances as by gases. We see then that 
the general relation of volume, i. e., 
1 
concentration, 
to pressure and temperature is the same for dis- 
solved substances as for gases. For either case 
may write : — 
Volume varies as pressure X temperature, or 
Volume = pressure X temperature X a con- 
stant quantity. • 
The most strilfing evidence in favor of the view 
that substances in solution are in a gaseous state is 
aftbrded by the fact that this constant has the 
same value for dissolved substances as for gases ; 
in other words, a dissolved substance at a certain 
temperature and which exercises on the solvent a 
certain pressure, occupies the same volume as it 
would if under these conditions of temperature 
and pressure it were in tlie form of a gas. An 
example will make this striking relation clear. 
At 0° C. and 760 m. m. of mercury pressure, the 
molecular weight of any gas expressed in grams 
occupies 22380 c.c. The pressure of 760 m. m. of 
mercury Is 1033 grams per square em., which 
number expresses the pressure in absolute units ; 
0° C. = 27.'} Oh the scale of absolute temperature ; 
therefore the value of the constant for gas is — 
1033 X 22380 
273 
84700 approximately. 
Sow take the case of sugar in solution. The 
molecular weight expressed l)y the formula 
C,2 ll-a Oil i* 342. In a 1 per cent, sugar solution 
therefore the molecular weight expressed in 
grams occupies .34200 c.c. At 0" C. the pressure 
on the solvent is found to be equal to 493 m.m. of 
mei'cury, that is to say 671 grains per square cen- 
timeter. The constant for sugar is therefore — 
671 X 342000 
273 
= 84200 
which agrees to within 6 parts in 1000 with that 
for gases. As this relation is not special to the 
case of sugar but is a general relation, it follows 
that Avogadro's law is obeyed by substances in 
solution, equal volumes of which, at the same tem- 
perature, and exercising the same pressure on the 
solvent, contain equal numbers of molecules. 
It is well known that the abnormal vapour 
densities of certain bodies which we now know to 
be dissociated in the gaseous state long opposed an 
obstacle to the acceptance of Avogadro's genera- 
lization. The exceptions met with in the case of 
substances in solution are of exactly parallel 
character. The deviations from the law are al- 
ways in one direction, giving too great a volume 
when the calculation is based on the received 
molecular weights. This is met by the hypothe- 
sis that such exceptional substances are to some 
extent dissociated in solution. As in the case of 
gases so also in the case of substances in solution, 
many bodies occupy a volume twice as great as 
that calculated from their molecular weight. 
These are regarded as being completely dissoci- 
ated into two constituents. The most important 
classes of substances which behave thus are the 
strong acids and bases and their Salts. A large 
number of phenomena afford evidence in support 
of this hypothesis of dissociation in solution, the 
study of which now constitutes an important 
field of work connected with the development of 
the new theory of solution. 
[American Analyst.] 
THE ALLEGED WORTH OF ALUMINIUM. 
In the Tribune of May 6 we find wiiat purports 
to be a comrtlunication fl'om a Springfield, Mass., 
correspondent, signing himself "C. M.," over the 
date of April 6, whifih traverses in no ambiguous 
phrases the generally accepted Value to the arti* 
and sciences of aluminium. He says : 
" It is interesting to watch certain crazes which 
at certain times in history have taken hold of ami 
still affect the minds of men. Thus the tulip 
craze, the South Sea craze, and the balloon craze 
in the beginning of this century and end of the 
last. In the present age we may notice the alu- 
minium craze. How people will iuVest tiiciusan(N 
of dollars on hearsay evidence, when they Cduld 
ascertain the ti-uth by buying twenty-five cents" 
worth of pure aluminium wire or sheet, belongs 
to the oddities of capital gone crazy. The yarns 
about aluminium are so numerous, and are re- 
peated in so many bold and frank lies, that it 
seems almost a vain task to try to set people 
right. Now, Jules Verne, the champion writer of 
scientific fancy, never uses aluminium much in 
his fiction. He knows too well that it is 'no 
good.' In ' De la Terre a la Lune ' he uses a largo 
aluminium bullet — not because of its strength, 
but because it is so light. Not long ago a cai'it 
from persons interested in the making of alumin- 
ium came to me, stating that aluminium is as 
hard as steel. All wrong! . The purer, the softer. 
Pure aluminium is just a trifle harder than oidi- 
nary cheap zinc or spelter, which can be soldered, 
threaded, and drilled without difliculty, while 
pure aluminium cannot be soldered reliahly, and 
for working is one of the ' nastiest ' metals in ex- 
istence. One great point in its favor, but only a 
point of skin-deep beauty, is that the aluiniuiuui 
rust forming in the air is white, while it is brow n- 
ish red on iron, green on copper, and gray on zinc, 
—but aluminium tarnishes readily, especially in 
salt air. Salt water acts upon it far more than it 
does upon iron, and a ship built of aluminimu 
would be full of holes after one trip around the 
globe. Therefore, for ship-building and all parts 
exposed to the water it is even intrinsically infe- 
rior to iron — therefore not worth six cents a 
pound. For roofing it is vastly inferior to gal- 
vanized iron, because it cannot be soldered, and 
much inferior to zinc— therefore not worth five 
cents a pouiul. For vessels to be used in the 
kitchen it could only be employetl if salt was 
kept away, but with salt present it would not 
have the slightest advantage over ordinary tin 
