=Special Uses of Calorimetric Pyrometers.=—The great drawback to the calorimetric method is that each observation necessitates a separate experiment, involving time and labour. The accuracy, moreover, is not comparable with that obtainable by the use of a thermo-electric or resistance pyrometer; and practically the only recommendation is the low initial cost of the outfit. When an occasional reading of temperature, true to 3 per cent., suffices, the calorimetric pyrometer may be used; and in special laboratory determinations the method will frequently be found of value. Considering the low cost of thermo-electric pyrometers at the present time, it is probable that the calorimetric method will be entirely superseded in industrial practice, as the former method gives a continuous, automatic reading, and is capable of furnishing records. Many firms have already replaced their “water” pyrometers by the more accurate and useful appliances now available. CHAPTER VIII FUSION PYROMETERS =General Principles.=—If a number of solids, possessing progressive melting points, be placed in a furnace and afterwards withdrawn, some may be observed to have undergone fusion whilst others would be unaffected. The temperature of the furnace would then be known to be higher than that of the melting point of the last solid melted, and lower than that of the first which remained intact. Taking, for example, a series of salts, the following might be used:- ══════════════════════════════════════╤═════════════════════ │ Melting Point. Salt. ├──────────┬────────── │Deg. Cent.│Deg. Fahr. ──────────────────────────────────────┼──────────┼────────── 1 molecule common salt + │ 650 │ 1202 1 molecule potassium chloride │ │ Common salt │ 800 │ 1472 Anhydrous sodium carbonate │ 850 │ 1562 ” ” sulphate │ 900 │ 1652 Sodium plumbate │ 1000 │ 1832 Anhydrous potassium sulphate │ 1070 │ 1958 ” magnesium sulphate │ 1150 │ 2102 ══════════════════════════════════════╧══════════╧══════════ If, on inspection, it were found that the sodium sulphate had melted, whilst the sodium plumbate had survived, the temperature of the furnace would be known to lie between 900° C. and 1000° C. If a number of salts or other solids could be found with melting points ranging between 900° and 1000°, it would be possible to obtain a reading within narrower limits. The accuracy of the method in all cases is decided by the interval between the melting points of successive test materials.