6S8 
CONVERSAZIONE. 
guides efiectually prevent any slipping. ^ Mr. T. Ross and Messrs. Smith and 
Beck contributed some of their best microscopes, and exhibited a number of 
very interesting objects during the evening, Mr. Casella sent some of the 
new mural standards, containing the metre and yard engraved on porcelain. 
This material has been selected, by the advice of the Chemical Society, as best 
suited to resist the influence of time and weather. Mr. Larkin exhibited some 
models of crystals, geometric figures, etc. ; and Mr. E. Blakeman showed some 
models for constructing glyptic formulse. This method of showing the consti¬ 
tution of chemical bodies was introduced by Dr. Hofmann in his lectures at the 
Royal Institution, and has since been adopted by Dr. Frankland and some 
others. A number of coloured balls, representing atoms, are connected together 
by arms, the latter representing the atomicities, or bonds, of the respective ele¬ 
ments. 
In the museum, two new engines were at work at intervals throughout the 
evening. One of these was the gas-engine invented by M. Hugon. It was 
exhibited by Mr. Caspar, the English agent, and appears to be a great improve¬ 
ment upon the one brought out some time back. No electricity is needed to 
work it, but the mixture of gas and air is forced by bellows into the cylinder, 
and is fired by a most ingenious arrangement of small gas flames. The great 
advantages possessed by this engine are that it can be started in a minute and 
as readily stopped. It requires no attention while it is at work, and there is 
no risk whatever, either from fire or boiler explosion. The other engine exhi¬ 
bited was the hot-air engine now being manufactured and sold by Edwards and 
Co., Limited. Many attempts have been made at different times by engineers 
to employ the expansive force of heated air as a motive power in the place of 
steam.. Such a principle promises great economy of fuel, and the Ericsson and 
other caloric engines were constructed in accordance with it; but they failed 
through several practical defects. The present hot-air engine differs from its 
predecessors mainly in the fact that the power, instead of being derived from 
the expansion of air heated in a separate generator, is obtained by the expan¬ 
sion of air heated by contact with the fuel itself, or, in other words, by the ex¬ 
pansive force of tlie heated products of combustion. This is accomplished by 
placing the fuel in a grate which can be hermetically closed, forcing the air re¬ 
quired for combustion into it by means of a pump worked by the engine itself, 
and compelling the heated gases from the fire to pass through the cylinder and 
lift the piston before they escape. The grate consists of a strong cylindrical air¬ 
tight iron vessel, lined with segments of fireclay. The fuel is introduced 
through the fire door, and the fire lighted in the ordinary manner, the fire-grate 
door being immediately closed, and the draught obtained through the ash-pit 
door. When the fuel is well ignited, the ash-pit door and the grate door are 
both closed air-tight, and secured by fastenings. The fly-wheel is then turned 
round two or three times by hand ; this, by working the air pump, forces a 
supply of air into the grate. It here comes in contact with the fire, supports 
the combustion, and also acquires great expansive force. It consequently presses 
on the piston in the cylinder, raises it, and the engine is set in motion. At the 
end of the stroke the expanded gases escape by a waste-pipe, which may be con¬ 
nected with an ordinary chimney. This engine is remarkably compact, one of 
two horse-power occupying a space of only six feet square, and as it requires 
no fitting it is portable. It requires no chimney to create a draught, the 
products of combustion being forced out. Consequently the heat in the waste 
gases may be readily utilized ; as, for instance, by conducting the gases in a pipe 
round an apartment to warm it, or for warming water by circulating the flue 
pipe through a tank, etc. The dominant advantage claimed for the engine 
however, is economy: one of two horse-power costs only a shilling per day for 
fuel. 
