[0<5tober, 
* Progress in Science . 
vacuum by means of a special apparatus, in which, instead of continuously 
rotating in one direction. as in the ordinary radiometer, the moving part is 
suspended by a glass fibre, which it twists in opposite directions, alternately. 
The movement is started by rotating the whole apparatus through a small 
angle, and the observation consists in noting the successive amplitudes of 
vibration when the instrument is left to itself, a mirror and spot of light being 
employed for this purpose. The amplitudes form a decreasing series, with a 
regular logarithmic decrement. The logarithmic decrement is nearly constant 
up to the point at which the vacuum is apparently equal to a Torricellian 
vacuum, the mercury in the gauge standing at the same height as a barometric 
column beside it ; but as the exhaustion proceeds beyond this point, the loga- 
rithmic decrement becomes smaller ; in other words, the amplitude diminishes 
less rapidly. By plotting the observations and supposing the curve continued, 
it is indicated that, if a perfect vacuum were attained, the logarithmic decre- 
ment would be zero, we should have perpetual motion with constant amplitude, 
and at the same time, the radiometer would cease to adt. He had tried other 
gases as well as air. Aqueous vapour is very unfavourable to the adtion of 
the radiometer; hydrogen, on the contrary, gave the best result of all. Prof. 
O. Reynolds and Mr. Schuster had published experiments which seemed to 
point to the true explanation of the adtion of the radiometer ; but he thought 
Mr. Stoney’s explanation the clearest. The molecules of the gas are beating 
about with the temperature; but, in tolerably dense air, they jostle one 
another, and their paths are short. When exhaustion is carried to'a certain 
high point, the molecules are sufficiently few, and their paths sufficiently long 
for them to rebound from the glass. 
We are compelled from want of space to omit notices of other valuable 
papers brought before Sedtion A. These, with a report of the work done 
in the other sedtions, will be given in our next number. 
Microscopy. — Mr. A. M. Edwards, of Newark, New Jersey, has experi- 
mented upon the properties of salicylic acid as a preservative for microscopical 
purposes. Casts of uriniferous tubules, obtained from a severe case of 
nephritis, mounted in 1874 in a dilute solution, are in as good condition as 
when first put up. Mr. Edwards has also succeeded with Volvox globcitur. 
Salicylic acid is worth further experiment, as our preservative fluids are as yet 
but imperfedtly understood, and researches upon their properties much needed. 
Labarraque’s solution, so frequently mentioned in various American 
methods of staining vegetable tissues, and used for bleaching preparatory to 
the dyeing process, is the “ Liquor Soda Chlorinata ” of the British 
Pharmacopoeia. 
Mr. William A. Rogers contributes to the “ Proceedings of the American 
Academy of Arts and Sciences” a paper “ On a Possible Explanation of the 
Method Employed by Nobert in Ruling his Test Plates.” Mr. Rogers has 
himself succeeded in ruling plates as fine as 80,000 lines to the inch. The 
paper contains three tables relating to the errors in spacing in plates ruled by 
Rutherford, Nobert, and the author, and a somewhat elaborate account of the 
mechanical contrivances employed in moving the plate to be ruled over given 
and equal spaces. The comprehension of this portion would be greatly aided 
by suitable illustrations. The tool used for ruling by Mr. Rutherford and Mr. 
Rogers is a diamond having a knife edge ; that used by Nobert is known only 
to himself. The stone is worked artificially to the required form, the use of 
the natural crystal, either whole or broken into chance fragments, having long 
ago been abandoned. In these experiments it has been discovered that glass 
has what may for want of a better term be described as a grain, fine, clean 
lines being only capable of being ruled in certain directions. It is a remark- 
able fadt that a properly-shaped diamond, set in the best position, is not at 
first capable of ruling the finest lines, but improves with use. When the 
diamond does its work perfectly the cut, even of the finest line, produces a 
sharp, singing sound, so that the operator can judge of the quality of the 
lines ruled almost as well by hearing as by sight. The best results have, 
