metallic sulphates, chlorides, etc., or mixtures of these, give results quite as good as those obtained with Seger pyramids. =Stone’s Pyrometer.=—This instrument is intended to indicate the correct temperature at which a metal or alloy should be poured, and consists of a silica tube at the bottom of which is placed an alloy melting at the temperature at which the material operated on should be poured. A silica rod rests on this alloy, and is connected at its upper end to an iron extension, the extremity of which engages a pointer moving over a scale. When the alloy in the silica tube melts, the rod falls through the molten mass and moves the pointer over the scale, thus giving a certain indication that the desired temperature has been attained. Arrangements exist for adjusting the pointer to zero at the commencement of a test. =Fusible Metals.=—Instead of clays or salts, a number of metals and alloys are sometimes used. These are placed in the form of short rods in numbered holes in a piece of firebrick and inserted in the furnace, and on withdrawal those which have undergone fusion will be seen to have taken the form of the holes in which they were placed. The temperature of the furnace is considered to lie between the melting points of the last of the series to undergo fusion and the first which remains unchanged. A series of metals of this description is more costly than clays or salts, but is more rapid in action, owing to the superior conductivity of metals. =Fusible Pastes.=—These consist of salts incorporated with vaseline or other suitable fat, and are used to detect the attainment of a specified temperature by a piece of metal. If, for example, it were desired to heat a piece of steel to 800° C. for a given purpose, a paste containing common salt might be smeared on its surface before placing in the furnace. On heating, the vaseline burns away, leaving a white mark due to the salt, and this white mark will be visible till the salt fuses. The disappearance of the white mark therefore indicates that the required temperature has been reached; and the method is simple and useful in cases where a number of articles are to be worked at a uniform temperature. CHAPTER IX MISCELLANEOUS APPLIANCES =Expansion and Contraction Pyrometers.=—Most substances, on heating, exhibit an increase in size, and on cooling return to the original dimensions. If, however, a chemical alteration occur during the heating, the resultant material may be permanently altered in size, so that on cooling the substance may be of less or greater dimensions than before. Both these phenomena have been applied to the measurement of high temperatures; the permanent shrinkage undergone by clay being utilised by Wedgwood in the instrument which was the first practical pyrometer; the expansion of a solid by Daniell, and of liquid by Northrup. Both forms are still in use to a limited extent, and will now be described. [Illustration: FIG. 68.—WEDGWOOD’S PYROMETER.]