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POPULAR SCIENCE REVIEW. 
of encounters between cloud-particles. If the contacts result in coalescence 
the drops must rapidly increase in size and be precipitated as rain. We may 
thus anticipate an explanation of the remarkable but hitherto mysterious 
connection between rain and electrical manifestations. 
Influence of Pressure on Spectra . — Herr Oiamician has contributed expe- 
riments on this subject to the Vienna Academy. The spectra of chlorine, 
bromine, and iodine show the same peculiarities under increased pressure ; 
the bright lines becoming broader, but not spreading into bands, a continuous 
illuminated background appears which often overpowers the lines. The 
spectrum of sulphur does not change at all, neither does that of phosphorus 
and arsenic. 
Metals behave very differently. Here a real band-like extension of the 
spectral lines takes place, while the continuous light remains subdued. In 
the mercury spectrum the enlargement of the green and violet lines is 
specially noteworthy. In the sodium spectrum the enlargement is very 
considerable, but can only be observed with the reversed or dark D line. 
Under high pressure sodium gives a continuous background in the imme- 
diate neighbourhood of the D line upon which the reversed line appears. 
The double line gradually merges into one, becoming wider until it finally 
covers the whole background. 
The Induction Balance and Sonometer . — Professor Hughes read a most 
important paper on these remarkable instruments before the Royal Society 
on May 15. After noticing Arago’s original discovery of the action of 
rotating plates upon a magnetic needle, as well as Babbage’s, Herschel’s, 
and Dove’s experiments, he remarked that in the microphone and telephone 
an instrument of extreme sensitiveness to minute induced currents had been 
obtained, and already published by him. In continuing the inquiry, the idea 
occurred to him that he might again investigate the molecular constructions 
of metals and alloys ; and with this object has obtained, after numerous 
comparative failures, a perfect induction balance, which is not only exqui- 
sitely sensitive and exact, but allows us to obtain direct comparative measures 
of the force or disturbances produced by the introduction of any metal or 
conductor. 
The instrument presented to the Royal Society consisted — 1st, of new 
induction-currents balance ; 2nd, microphone, with a clock as a source of 
sound ; 3rd, electric sonometer, or absolute sound measurer ; 4th, a receiving 
telephone and three elements of Daniells’ battery. 
In order to have a perfect induction-currents balance suitable for physical 
research, all its coils, as well as the size and amount of wire, should be 
equal. The primary and secondary coils should be separate, and not super- 
posed. The exterior diameter of the coils is 5£ centims., having an interior 
vacant circular space of 3| centims. j the depth of the flat coil or spool is 
7 millims. 
Upon this box-wood spool are wound 100 metres of No. 32 silk-covered 
copper wire. Four of such coils, formed into two pairs, are used, the 
secondary coil being fixed permanently, or by means of an adjustable slide, 
at a distance of 5 millims. from its primary ; on the second similar pair 
there is a fine micrometer screw, allowing the adjustment of the balance to 
the degree of perfection required. 
