Natural-Philosophical €ollections. 307 
more clear parts are land covered with snow. He thinks that the places sur- 
mounted by a central cone are extinct volcanoes, seeing that no clouds are per- 
ceivable above the surface of the moon. The snow and ice with which it is en- 
veloped explain, as he imagines, the cause of the brightness of its atmosphere, 
or the absence of an atmosphere. He conjectures that this vast accumulation 
of ice and snow which exists at the surface of the moon may be deduced from the 
nature of its revolutions. He offers to construct instruments of the same kind as 
he has used, by means of which numerous phenomena may be observed, at the 
price of from 50 to 100 dollars, and to furnish, at the same rate, solar microscopes 
made after a new principle, and so powerful, that, at a distance of twelve feet, they 
magnify 5,184,000 times.—( Lond. & Paris Observ. Mars 1829. )—Bull. des 
Sciences Physiques. 
On Maximum and Minimum Thermometers. From the Course of Natural 
Philosophy and Chemistry, by Mr. Lechevallier, Lieutenant of Artillery, read 
to the Academy af Mentz, the 4th January 1829.—In a great number of scientific 
researches, and more especially in those which relate to meteorology, it is often 
necessary to know the maximum and minimum temperature during a given time. 
The construction of instruments destined to preserve the indications of mere tem- 
peratures, and which are called maximum and minimum thermometers, has exer- 
-cised the ingenuity of many philosophers, amongst whom we may quote Six, Bel- 
lani, Rutherford and Mr. Gay Lussac. Among the maximum and minimum 
thermometers which they have proposed, and which are all based on the dilatation 
of liquids, that of Rutherford appears to have obtained a marked preference. It 
consists of two thermometers laid horizontally, the one with quicksilver and a 
steel index, the other with spirits of wine with an ivory index, for the minimum 
temperatures. Nevertheless, it appears that this instrument is not exempt from 
all inconveniences ; it has been found fault with, in as much as the indexes do 
not always remain in the places where the liquids have brought them to, and that 
the separation of the column of liquid, which may accidentally result from the 
necessity in which we are of employing tubes whose diameter is not very small, 
and which, by leaving air in the interior, may give rise to false indications. Be 
it as it may, a very exact minimum and maximum thermometer may be procured, 
by making a slight addition to the metallic thermometer of Breguet. A circular 
or concentric hole must be made in the centre of the graduated circle, which marks 
the course followed by the point of the needle. Two small and light moveable 
bodies must be placed on each side of the needle, so that they may yield to the 
slightest effort. It is evident, then, when the needle moves in any direction, that 
it will push one of these before it, and that this mobile body remaining at the 
point, where the needle left it, will indicate a maximum or minimumte mperature 
according to the direction of the needle. To experiment on this instrument, it 
suffices to approach the two mobile bodies so that they touch the needle. 
On the Solidification of Plaster, by M Gay Lussac.—Every. one knows the 
property which plaster possesses, when deprived of its water by heat, of becoming 
solid with that fluid. The consistency which it acquires is very variable, and the 
purest plasters are precisely those which acquire least hardness. The cause has 
been attributed, in Paris plaster, to the presence of a few hundredths of carbonate 
of lime; but, without doubt, erroneously ; for the heat necessary to bake the 
plaster is, in the small way, not above 300° F., and, in the large way, is never 
earried to the degree necessary to decompose the carbonate of lime. Besides, 
¢alcined plaster rarely contains free lime, and the addition of that base to those 
plasters, which have but little consistence, does not sensibly improve them. I 
think that we must search for the difference of consistency, which is acquired by 
different plasters, when mixed with water, in the hardness which they possess in 
their natural uncalcined state; a hardness which we cannot explain, but must 
take as a natural fact. That stated, I suppose that a hard plaster-stone, having 
lost its water, will acquire greater consistency when returning to its first state than 
