barus.] METHODS OF PYROMETRY. 33 
an electro-magnetic engine. The mechanism of Crafts' new thermome- 
ters appears to be particularly }>erfect in this respect. An air ther- 
mometer in which the pressure is directly measured manometrically is 
described by Codazza. 1 A rigorous investigation of the formula of the 
air thermometer, with a view toward the construction of an apparatus 
of exceptional delicacy, has lately been made by Grassi. 2 Finally, the 
possible condensation of gases on metallic air thermometers of very 
large internal surface has been incidentally discussed by Fuess. 3 Op- 
erating with bulb in form of a cylindrical ring, Fuess found that for a 
correct ice point the boiling point of water showed a value enormously 
high, which gradually decreased without reaching a normal value. His 
research is unfinished, however, and thus the full interpretation of these 
anomalous results is yet to be given. 
After the earlier work of Deville and Troost and the papers of Schinz, 
the most important memoir on high temperature measurement was 
published by Weinhold. 4 Having discussed the important methods of 
empirical pyrometry, with reference to their availability for practical 
work or for research, Weiuhold uses his air-thermometer for a rede- 
termination of the boiling point of zinc. Unfortunately his high value, 
1,035° at 71.89 cm , which is only a little below the erroneously large 
values of Deville and Troost, casts a slur over much of Weinhold's 
elaborate experimentation, and his criticism on the merits of Siemens' 
pyrometer, of calorimetric pyrometers, and of the dissociation pyrome 
ters fail to obtain the consideration which they probably deserve. 
Weinhold's bulb is of Meissen porcelain and his instrument of measure- 
ment is a modified Jolly thermometer. 
In this place it is well to call attention to certain experiments com- 
menced at about this time by Amagat and others to test the correctness 
of B03 le's law at different temperatures and high pressures. The con- 
stants hitherto adopted in high temperature air thermometry for all 
temperatures and pressures indiscriminately were those investigated 
by Kegnault 5 and by Magnus. 6 By Amagat, 7 Cailletet, 8 and others 
these researches were pushed to great nicety for pressures as high as 
Codazza: Dingler's Jour.,, vol. 210, 1873, p. 255. 
a Grassi : Rend, dell' Academia delli Scienze fisiche e math., vol. 24, pp. 16, 131, 1885. 
Bei blatter, vol. 10, 188G, p. 387. 
3 Fuess: Zeitschr. fur Iustruraenteuk., vol. 5, 1885, p. 2'74. 
4 Weinhold: Osterprogramui der hob. Gewerbesch. zu Chemnitz, 1873; Pogg. Ann., 
vol. 149, 1873, p. 186. 
6 Regnault: Relation des experiences, Paris, 1847, pp. 15, 168. 
6 Magnus: Pogg. Ann., vol. 55, 1842, p. 1. 
7 Amagat: Fortschr. d. Phys., 1869, p. 155 ; C. R., vol. 71, 1870, p. 67 ; C. R., vol. 73, 
1871, p. 183; Archives sci., phys. et nat. Geneve, 2d series, vol.40, 1871, p. 320; Ann. 
ch. et Phys., 4th series, vol. 29, 1873, p. 246 ; ibid., 5th series, vol. 22, 1881, p. 353, etc.; 
C. R., vol. 94, 1882, p. 847; C. R., vol. 95, 1882, p. 638; C. R., vol. 99, 1884, pp. 1017, 
1153; C.R., vol. 103, 1886, p. 429. 
8 Cailletet: C. R., vol. 70, 1870, p. 1131. 
Bull 51 3 
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