SCIENTIFIC SUMMARY. 
281 
observed with an eye-piece magnifying twelve times, in whose focus is 
placed a bar moveable by a micrometer screw. Mr. Rutherford believes 
that the following defects exist in this apparatus : — 1st. Absence of achro- 
matism in the great condenser, on account of which only a small portion 
of the spectrum can be examined at a time. 2nd. Too much power in the 
observing telescope. 3rd. The uncertainty of bringing the micrometer 
bar and lines into contact, there being no illumination. 4th. The want 
of a flame-line in the field of view, to insure the detection of slight dis- 
placements of the spectrum. Professor Airey’s instrument is objected to, 
on the score that the lines are only rendered visible by the existence of 
aberrations, which prevent fine definition. Secchi’s spectroscope is a far 
more perfect one than either of the two former. The light of the great 
Roman equatorial passes through a slit at the focal point, then traverses a 
cylindrical lens, is next rendered parallel by another lens, and finally falls 
upon the prism, the latter is composed of four flint-glass prisms of 90°, 
cemented to five crown-glass ones of the same angle. The only objection 
to this form is the great thickness of the glass in the prisms, the con- 
sequent absorption of light, and the increased danger of heterogeneity of 
material. The author terminates his interesting paper by attributing the 
shortening of the spectrum, seen by Mr. Glaisher at a height of four and a 
half miles, to the absence of light, and also to the monochromatic nature 
of the blue sky, which at that altitude held but little vapour capable of 
reflecting white light. 
Sound produced by Electricity.— Professor Thomson has recently observed 
a very interesting and peculiar phenomenon in connection with the dis- 
charge of a condenser through a galvanometer coil. In experimenting on 
the electrostatic capacities of some specimens of insulated wire intended 
for submarine telegraphic cables, he had occasion to discharge through a 
galvanometer coil a condenser consisting of two parallel plates of metal, 
separated by a space of air] of about *007 inch across, and charged to a 
difference of potentials equal to that of about 800 Daniell’s elements. At 
the moment of each discharge a sharp and slightly prolonged sound was 
observed, which seemed to come from the interior of the case containing 
the condenser. It was at first thought that it resulted from the passage 
of a spark across the air-space of the condenser ; but this w T as found not 
to be the case. Several other experiments were conducted with a view to 
ascertain whether a spark had been produced within the condenser, 
but all showed that no such development occurred. Hence the observer 
concludes that a plate of air emits a sound on being suddenly subjected to 
electric force , or on experiencing a sudden discharge of electric force through it. 
— Chemical News , October 24th. 
Necessary Conditions of Molecular Motion. — The conditions under which 
the phenomena of molecular movements manifest themselves are various, 
and may be thus classified : — 1st. In the solution of substances for crystal- 
lization or dialysis. 2nd. When the substance is melted, as in the pro- 
cess for purifying lead and antimony. 3rd. When the substance is subject 
to mechanical violence, as in the conversion of fibrous malleable iron into 
the brittle and crystalline variety by means of concussions. 4th. During 
chemical action. But few instances of this class have been yet recorded. 
