274 Progress in Science. (April, 
and distilled water. Put the jelly into test-tubes 3 inch in diameter, and when 
you wish to mount a slide warm the upper part of the tube: in this way you 
can pour out any quantity free from bubbles. It is perhaps well to put a trace 
of varnish, or some essential oil, on the corks, lest they should get mouldy. 
The chloride of barium prevents fungi in the jelly, and is the best preservative 
we know of; but something is absolutely necessary. By all means avoid 
putting alcohol, creosote, &c., in the jelly, as they dissolve varnishes, and also 
spoil the colour of some objects.”” The hints on mounting in this paper are of 
great value. 
An object-glass of American manufacture has arrived in London, engraved 
‘““R. B. Tolles, Boston. Immersion, 3th. 180°!! Balsam angle, 98.” It 
performs well, defining splendidly, but is wanting—as are some good English 
objectives—in flatness of field. Why stop at 180°? Surely an enlightened 
citizen of a free and independent country will not be trammelled by the laws 
of mathematics and optics. 
Heat.—In a paper communicated to the Royal Society, ‘* On the Aétion 
of Heat on Gravitating Masses,’ the Editor of this Journal has recorded 
experiments which arose from observations made when using the vacuum- 
balance, described by the author in his paper ‘‘On the Atomic weight of 
Thallium,” * for weighing substances which were of a higher temperature 
than the surrounding air and the weights. There appeared to be a diminution 
of the force of gravitation, and experiments were instituted to render the 
action more sensible, and to eliminate sources of error. After an historical 
résumé of the state of our knowledge on the subject of attraction or repulsion ~ 
by heat, the author describes numerous forms of apparatus successively more 
and more delicate, which enabled him to detect, and then to render very 
sensible, an action exerted by heat on gravitating bodies, which is not due to 
air-currents, or to any other known force. The following experiment with a 
balance made of a straw beam with pith-ball masses at the ends enclosed in 
a glass tube, and connected with a Sprengel pump, may be quoted from the 
paper :—‘‘ The whole being fitted up as here shown, and the apparatus being 
full of air to begin:with, I passed a spirit-flame across the lower part of the 
tube at b, observing the movement by a low-power micrometer ; the pith-ball 
(a, b) descended slightly, and then immediately rose to considerably above its 
original position. It seemed as if the true action of the heat was one of 
attraction, instantly overcome by ascending currents of air. . . . 31. In order 
to apply the heat in a more regular manner, a thermometer was inserted in a 
glass tube, having at its extremity a glass bulb, about 1} inches diameter; it 
was filled with water, and then sealed up... . The water was kept heated to 
70° C., the temperature of the laboratory being about 15°C. 32. The baro- 
meter being at 767 millims., and the gauge at zero, the hot bulb was placed 
beneath the pith-ball at'b. The ball rose rapidly; as soon as equilibrium was 
restored, I placed the hot-water bulb above the pith-ball at a, when it rose 
again, more slowly, however, than when the heat was applied beneath it. 
33. The pump was set to work, and when the gauge was 147 millims. below 
the barometer, the experiment was tried again; the same result, only more 
feeble, was obtained. The exhaustion was continued, stopping the pump from 
time to time, to observe the effect of heat, when it was seen that the effect of 
the hot body regularly diminished as the rarefaction increased, until when the 
gauge was about 12 millims. below the barometer the action of the hot body 
was scarcly noticeable. At ro millims. below it.»was still less; whilst when 
there was only a difference of 7 millims.. between the barometer and the 
gauge, neither the hot-water bulb, the hot rod, nor the spirit-flame caused the 
ball to move in an appreciable degree. ‘The inference was almost irresistible 
that the rising of the pith was only due to currents of air, and that at this 
near approach to a vacuum the residual air was too highly rarefied to have 
power in its rising to overcome the inertia of the straw beam and the pith 
balls. A more delicate instrument would doubtless show traces of movement 
* Phil. Trans., cxliii., part 1, p. 277. 
