Alcohol ought to be very pure and well rectified. It is necessary to colour it, because, being colourless of itself, it could not be seen in capillary tubes. To colour alcohol, you infuse carmine in it, and, after some time, decant or filter the clear solution. The liquid should be perfectly transparent, and free from all extraneous substances. It is not proper to employ alcohol in the construction of standard thermometers; mercury being much preferable.
Sulphuric Acid.—It is made use of for the differential thermometer, and the thermoscope of Rumford. It has the advantage of being lighter than mercury, and very slightly volatile: these two qualities, joined to its tendency to absorb the vapour of water, render it very proper to be employed for various instruments. It must be very concentrated, and tinged red by carmine.
Ether.—Sulphuric and nitric ether, with which some small instruments are filled, are merely employed to shew with what facility these liquids are brought to their boiling point.
Of Graduation in general.—Graduation, generally speaking, consists in dividing lines, surfaces, and capacities, into a certain number of equal or proportional parts. It is not our intention to treat here of the methods furnished by practical geometry for effecting such divisions with mathematical accuracy; these methods are known to every body. We shall confine ourselves to describing the processes of graduation which are peculiar to the instruments constructed by the glass-blower.
Examination of the Bore of Tubes.—We have already observed, that, for standard thermometers and other instruments which require to be made very accurate, it is necessary to employ tubes which are extremely regular in the bore. When a drop of mercury, passed successively along all parts of the tube, forms everywhere a column of the same length, the examiner is assured of the goodness of the tube.
That a tube may be regular in the bore, it is not necessary that the bore be cylindrical; it is sufficiently accurate when equal lengths correspond to equal capacities. A tube with a flat canal, for example, can be perfectly accurate without at all approaching the cylindrical form. It is only necessary that a drop of mercury occupy everywhere the same length. We may observe, by,the way, that, in flat canals, the flattening should be always in the same plane.
Division of Capillary Tubes into parts of equal capacity.—As it is very difficult to meet with capillary tubes which are exactly regular in the bore, it happens that the tubes which glass-blowers are obliged to employ have different capacities in parts of equal length. You commence the division of these tubes into parts of equal capacity by a process described by M. Gay-Lussac. You introduce a quantity of mercury, sufficient to fill rather more than half the tube, and make a mark at the extremity of the column. You then pass the mercury to the other end of the tube, and again mark the extremity of the column. If you so manage that the distance between the two marks is very small, you may consider the enclosed space as concentric, and a mark made in the middle of the division will divide the tube into two parts of evidently equal capacity. You divide one of these parts, by the same process, into two equal capacities, and each of these into two others; and in this manner you continue to graduate the tube until you have pushed the division as far as you judge proper.