530 



NA TURE 



[Sej'tember 30, 1897 



on the thermal expansion of Bayeux porcelain accompanied 

 these researches which, undertaken together with M. Gosse of 

 the Bayeux works, occupied them intermittently for about seven 

 years. A full summary of their data did not appear, however, 

 until 1880, when, together with a new vacuum method of high 

 temperature air thermometry, they communicated the results of 

 twenty-seven measurements on the boiling point of zinc. Their 

 new results are in good accord with the data of Becquerel, already 

 cited, and the more recent results of VioUe and others for the 

 same landmark in the region of high temperatures. Measure- 

 ments between o" and 1500° had thus reached a degree of 

 precision of about 15° in 1000", 



The further development of pyrometry took a somewhat 

 different direction. Regnault (1861) had already made use of a 

 displacement method, in which the measuring gas is removed 

 bodily into the measuring apparatus by an absorbable gas. But 

 the method was independently revived by Prof. Crafts, of the 

 Boston Institute of Technology. These methods are not of 

 especial excellence below 1500' ; but above that temperature, 

 when most solids tend to become viscous, their importance 

 increases (as Crafts duly pointed out) in proportion to the 

 rapidity with which the measuring operations can be completed. 

 One or two minutes may suffice, and different gases may be tested 

 consecutively. It is in this way that Victor Meyer and his 

 pupils, after demonstrating the dissociation of iodine and chlorine 

 molecules, succeeded in penetrating quantitatively to very much 

 less accessible heights of temperature. A particular desideratum 

 was a rigid test as to the stability of the molecule of the standard 

 measuring gases (oxygen, hydrogen, nitrogen). The results 

 were favourable inasmuch as for these and for many gases, like 

 COo, SOo, HCl, lig, &c., the expansions obtained were linear 

 functions of each other. 



In their final work, temperatures as high as 1700° were 

 reached, the air thermometer for this purpose being tubular in 

 form, consisting of very refractory fire-clay with an interior and 

 exterior lining of platinum and with two end tubulures of 

 platinum for influx and efilux of gases. Among many results of 

 great chemical interest their researches showed that metallic 

 vapours, phosphorus, sulphur, &c., at high temperatures tend to 

 pass into the monatomic or the diatomic molecular structure. 



Some time after (1887) a series of experiments furthering the 

 line of research of Deville and Troost were made with a geologic 

 aim in view in the laboratory of the U.S. Geological Survey. 

 Finally, porcelain air thermometry was taken up with great 

 vigour by the Reichsanstalt. These results, due to Holborn 

 and Wien, are now almost exclusively quoted, and carry the 

 stamp of the great institution from which they emanated. They 

 have been wisely made commerically available by the deposition 

 with Heraus in Hanau (Germany) of a platinum rhodium alloy 

 definitely calibrated for a temperature range of 1400°. 



Apart from this, these researches contain no essential novelty 

 except, perhaps, a more detailed attempt to investigate the stem 

 error of the thermometer bulb; their procedure otherwise is ident- 

 ical with the method developed in America. I am not therefore 

 inclined to yield to it the unhesitating deference which has become 

 customary. There can be no doubt, in view of the splendid facili- 

 ties due to the co-operation of the Royal Prussian Porcelain works 

 — facilities which those who have been baffled by porcelain tech- 

 nology, or have had to coax unwilling manufacturers into re- 

 luctant compliance, will appreciate — that the data of the 

 Reichsanstalt will eventually be standard. For the present, 

 however, I should be more impressed by some sterling novelty 

 either in the direction of a larger range of measurement, or of 

 method. Conceding that an accuracy of 5° at 1000° has been 

 reached, all results above 1500° remain none the less subject 

 to increasingly hazardous surmise. 



A beautiful method of absolute thermometry, albeit as yet 

 only partially developed, is due to Topler. In this the den- 

 sities of communicating columns of gas are compared very much 

 as in Dulong and Arago's classical method for liquids, by the 

 gravitation pressures which correspond to these unequally hot 

 columns. To accomplish such extremely fine-pressure measure- 

 ment, Topler invented the " Druck libelle," an inversion, as it 

 were, of the common level, in which therefore the motion of 

 the bubble (or of a thread of liquid) indicates a change of 

 pressure conditioned by the invariable horizontality of the 

 instrument. 



The development of the practical forms of continuous intrinsic 

 thermoscopes (the radiation, the ther77io-electric, and the electric 

 resistance methods) went more or less hand in hand with the 



NO. 1457. VOL. 56] 



development of the air thermometer, although the latter is de- 

 cidedly the more recent. Aside from pioneering experiments of 

 Miiller (1858) ard others, the well-known Siemens resistance 

 pyrometer (1871) was the first instrument in the field. It was 

 based upon data obtained from platinum, copper and iron, by 

 the calometric method of calibration. This instrument has 

 been remarkably perfected by Callendar and Griffiths, using 

 specially pure platinum calibrated by comparison with the air 

 thermometer as far as about 600°. Notwithstanding these 

 improvements the resistance pyrometer is inferior in my judg- 

 ment to the thermo-electric pyrometer on account of the greater 

 bulk and fragility of the exposed parts, and the tendency of 

 platinum to waste itself gradually at high temperatures. Its 

 upper limit of temperature measurement is thus limited ; for even 

 if the difficiilty of selecting suitable terminals for the coil is set 

 aside, the difficulty of finding an insulator at very high tempera- 

 tures would remain. According to Holborn and Wien resist- 

 ance is seriously subject to the influence of furnace gases, and 

 permanence of the low temperature constants does not imply a 

 like permanence of the high temperature constants of the metal. 



Radiation pyrometry, curiously enough, is the most vener- 

 able method within the whole scope of the subject. It was 

 introduced by Newton (1701) in Vm scala graduum caloris, in 

 connection with his well-known law of cooling. Not to 

 mention minor workers, it was successively attacked and re- 

 vived in most of the noteworthy high temperature investiga- 

 tions. Pouillet and Draper have studied it ; Becquerel, Crova, 

 Violle, Le Chatelier, Langley, Nichols, Paschen and others 

 have advanced it. It remains to-day the most promising, as 

 well as puzzlingly fascinating, subject for pyrometric research. 

 One need merely advert to its broad scope in relation to the 

 temperature of the heavenly bodies to acknowledge this. Here 

 I can only allude to Becquerel's principle that the radiation of 

 opaque bodies is spectrometrically alike at the same temperature, 

 a result which hasCrova's more recent assent ; to Violle's photo- 

 metric measurements of the total emission of platinum ; to the 

 more recent work in the same direction of Violle andLe Chatelier, 

 in which consistent results were obtained for oxide of iron and 

 platinum as far as 1500° to 1700° ; to Stefan's law, as proved by 

 Boltzmann and the variety of discussion it has elicited ; to H. F. 

 Weber's collateral equation ; to the Johns Hopkins measure- 

 ments, &c. Another school of observers, including Langley, 

 Paschen, and others, has undertaken the promising but much 

 more laborious method of bolometric measurement of the dis- 

 tribution of spectrum energy in its relation to temperature. 

 Without doubt, however, the whole subject is yet in priniis 

 rudimentis ; the results are confessedly "intrinsic." Indeed, 

 vagueness in the nature of the radiating source lowers with 

 sufficiently threatening aspect to chill the fondest hopes. When 

 one is told by Violle, working on Mont Blanc, that the tempera- 

 ture of the sun is 2500° ; thereupon by Rossetti that it is 9965°, 

 by Le Chatelier that it is at least 7600°, by Paschen that it is 

 below 5000°, by Wilson and Gray that it is 8000°, &c., one 

 wisely concludes that more may yet be learned about it. Our 

 sympathies naturally go with those who, like Lummer and 

 Wien and the Johns Hopkins people, are beginning funda- 

 mentally with the search for an absolutely black body. Less 

 superstructure and more sub-cellar is perhaps the watchword in 

 radiation pyrometry. 



Turning now to the last and most important of the methods of 

 practical pyrometry, we find a curiously meandering evolution ap- 

 parent. I have already indicated that Pouillet (1836) was the 

 first to complete a legitimate piece of calibration work. Pouillet 

 might have condemned the method, but for some reason Tail's 

 thermo-electric anomalies of red-hot iron were not detected. 

 Regnault (1847), who was the next to take up the subject as it 

 happened with the same couple, made this condemnation sweep- 

 ing enough. It was not the real perversity of the platinum-iron 

 couple which provoked Regnault, for of this neither he nor 

 Pouillet became aware. Regnault's objection (as we should put 

 it to-day) lay in the fact that the thermo-couple obeyed Ohm's 

 law, which in that early day lay somewhat beyond the great 

 physicist's range of interest. Fortunately, but none the less long 

 after, Becquerel followed with his palladium and divers platinum 

 couples, carefully calibrated and efficiently used. What these 

 platinum couples were is not stated. They cannot have been 

 very sensitive, or they would have been preferred to the pal- 

 ladium-platinum couple. Indeed, the metallurgy of platinum 

 alloys did not reach a degree of refinement until Deville and 

 Debray (1875) overhauled the chemical separation of platinum 



