SCIENTIFIC SUMMARY. 
451 
oxygen. Tliey would form water, and still leave a pair of atoms, or a single 
molecule of hydrogen attached to the platinum, — Vide Proceedings of the 
Royal Society, No. 103, vol. xvi. 
Value of the Aneroid Barometer. — Mr. Balfour Stewart lately addressed 
a paper to the Boyal Society, in which he gave an account of certain ex- 
periments on aneroid barometers made at the Kew Observatory. These 
experiments were of the highest interest, and they led to the following con- 
clusions : — (1) A good aneroid of large size may be corrected for tempera- 
ture by an optician, so that the residual correction shall be very small. 
(2 a) If an aneroid correct, to commence with, be used for a balloon or 
mountain ascent, it will be tolerably correct for a decrease of about 6 inches 
of pressure. (2 /3) A large aneroid is more likely to be correct than a small 
one. (2 y) The range of correctness of an instrument used for mountain 
ascents may be increased by a previous verification, a table of corrections 
being thus obtained. (3 n) If an aneroid have remained some time at the 
top of a mountain, and be supposed correct to start with, then it will give 
good results for about 8 inches of increase of pjessure. (3 (3) If the aneroid 
has been previously verified, it is likely to give a better result. (4) After 
being subjected to sudden changes of .pressure the zero of an aneroid gradually 
changes, so that under such circumstances it ought only to be used as a 
differential and not as an absolute instrument, that is to say, used to deter- 
mine the distance ascended, making it correct to begin with, or to ascertain 
the distance descended, making it correct to begin with, it being understood 
that the instrument ought to be quiescent for some time before the change 
of pressure is made. 
Influence of Pressure on the Combustion of Gases. — Dr. Frankland has 
been trying the effect of pressure on the combustion of hydrogen and 
carbonic-oxide in oxygen, and has found very remarkable results. The 
appearance of a jet of hydrogen burning in oxygen is known to everyone. 
On increasing the pressure to two atmospheres, the light is much increased ; 
but on increasing it to ten atmospheres, the light emitted by a jet an inch 
long is amply sufficient to enable the observer to read a newspaper at a 
distance of two feet from the flame, and this without any reflecting smTace 
behind the flame. Seen with the spectroscope the spectrum of the flame 
is bright and perfectly continuous from red to violet. The carbonic oxide 
is still more intense under similar circumstances. 
Effect of Nuclei in Crystallisation, — Mr. Tomlinson’s conclusions on this 
interesting point may be thus summed up : — (I) That a number of hydrated 
salts form supersaturated solutions, and remain so even at low temperatures 
simply from the absence of a nucleus to start the crystallisation. (2) That 
a nucleus is a body that has a stronger adhesion for the salt than for the 
water which holds the salt in solution, a state of things brought about by 
the absence of chemical purity. (3) That three or four salts form super- 
saturated solutions, which in cooling down deposit a modified salt or one of 
a lower degree of hydration than the normal salts. (4) That this modified 
salt is formed first by the deposit, in small quantity, of the anhydrous salt, 
which entering into solution, forms a dense lower stratum containing less 
water than the rest of the solution, in which lower stratum the chemical 
purity can be reduced to low temperatures without crystallising. Mr. Tom- 
