February 27, 1902] 



NA TURE 



591 



The Origin of the Scale of Fahrenheit's Thermometer. 

 VorR issue of February 13 contains, on p. 348, a note on 

 the above subject, in which it is stated that Fahrenheit based 

 his scale upon a scale previously adopted by Newton, Newton's 

 scale having its zero at freezing point and the temperature of the 

 human body marked as 12 degrees. Fahrenheit (says .Sir Samuel 

 Wilks) found Newton's divisions too large. He therefore 

 divided them by two. Next he altered his zero to the tem- 

 perature produced by a mixture of ice and salt. Later on he 

 again divided each degree into four parts, giving the scale 

 which is now in use. This explanation is substantially that 

 which is given in the " Encyclopx-dia Britannica." 



It is evident that the origin of the Fahrenheit scale is a matter 

 of some speculation. A recent work, the "Evolution of the 

 Thermometer," by Mr. H. C Bolton (reviewed in Nature of 

 May 9, 1901), states that Fahrenheit's selection of a scale was 

 unfortunate, and did not appear to have been based on anything. 



It seems very unlikely that Fahrenheit, who was an accom- 

 plished man of science and experimenter, and whose ther- 

 mometers were acknowledged to be a great advance on others 

 existing at the time, should have based his scale on nothing at all. 



An examination of the main features of Fahrenheit's work 

 upon thermometers gives, I think, the key to the origin of the 

 scale, and shows that he based it upon a very sound and 

 scientific foundation. In discussing this question, one must 

 have a regard for the state of the knowledge of kindred matters 

 at the beginning of the eighteenth century, and consider how 

 the problem would be likely to present itself to Fahrenheit. 



Reference is made in the note to a paper in the Philosophical 

 TraHsactions for 1701, supposed to have been written by 

 Newton. In this paper, which is written in Latin, is described 

 a scale of degrees of temperature (Scala graduum Caloris) from 

 the freezing point of water to the melting point of gold, but it 

 does not appear that this scale was intended to be actually 

 applied to a thermometer. It seems only to be intended as a 

 convenient scale of reference for comparing temperatures cover- 

 ing a very wide range. The zero or starting point is the 

 freezing point of water. The external temperature of the human 

 body is taken as the second point from which the scale is 

 derived. The range of temperature between these two points is 

 divided into twelve parts. The freezing point is, therefore, 

 called o, and the body temperature 12. The scale is continued 

 upwards, and it was found that the temperature of water boiling 

 violently corresponded to 34 degrees. Many other degrees are 

 noted as indicating the melting points of metals, &C. 



The paper continues with a description of a thermometer, the 

 liquid element of which is linseed oil. The actual scale of the 

 thermometer, however, was not that described above, but was 

 determined as follows : — 



The thermometer was placed in melting snow. The space 

 filled by the oil in the bulb and the stem together was taken as 

 occupying 10,000 parts. The same oil, when expanded by the 

 heat of the human body, occupied a space of 10,256 parts, and 

 by the heat of boiling water 10,725 parts. Thus, on this ther- 

 mometer, if the freezing point was marked o, body temperature 

 was 256 and boiling water 725. It was by means of this ther- 

 mometer that the temperatures were obtained from which the 

 "Scala graduum Caloris" was computed. 



r.ahrenheit is credited with having been the first to use 

 mercury in the thermometer. He also discovered how to pro- 

 duce a temperature much below the freezing point of water by 

 mixing " ice, water and sal-ammoniac or salt." 



In a paper (also in Latin) which he contributed to the 

 Philosophical Transactions of 1 724, on the subject of" Experi- 

 ments concerning the Freezing of Water,"' he described his 

 thermometer, but did not explain his reasons for adopting the 

 particular scale. It may be safely assumed that he was ac- 

 quainted with the paper published in 1701 referred to above. 



Having then decided upon the use of mercury in his ther- 

 mometer in place of the oil previously used, the problem upon 

 what basis his scale should be constructed would next arise. 

 What could be more natural than to base it upon the expansion 

 of mercury itself? The idea of making his degree or unit that 

 difference of temperature by which the liquid expands by one 

 ten-thousandth part of its volume would naturally occur to him, 

 for it had already been done in the case of the oil thermometer. 

 That this is the basis of the Fahrenheit scale I think is proved 

 by the fact that for each degree of the Fahrenheit scale mercury 

 does expand by one ten-thousandth part of its volume. 



Having, therefore, determined upon the size of his divisions 



or degrees, the next thing was to fix on a zero or starting point. 

 What, again, could be more natural than to start with the 

 greatest degree of cold which he knew how to produce, namely, 

 the temperature of the ice and salt mixture ? Having settled 

 upon this, everything else follows, and we have the Fahrenheit 

 scale as we know it to-day. The thermometer registers for 

 freezing point 32°, blood heat g8J°, and boiling point z\z\ 

 In his own description of his thermometer, he states that the 

 temperature of the body is 96°, but this slight error was pro- 

 bably due to the thermometer not being properly heated by 

 that part of the body to which it was applied, and in any case 

 does not affect this explanation, which, I think, suggests that 

 the Fahrenheit scale is based upon scientific principles, and 

 is not, as is often supposed, a scale without rhyme or reason. 



Gilbert S. Ram. 



The Inheritance of Mental Characters. 



I QtJiTE agree with Prof. Cockerell that further discussion of 

 this subject had better be postponed, if, indeed, it be not wholly 

 unprofitable. But I may, perhaps, be permitted to make three 

 remarks : — 



(i) The coefficient of correlation is a measure of the degree 

 of resemblance between brothers. We are told it may be due 

 to "soul," heredity or environment. "Soul," I take it, can 

 only contribute to likeness between brothers, if they have like 

 "souls." If so, I suppose the likeness of "soul'" is due to 

 inheritance of " soul,' and I do not see how this is going to be 

 disiinguished from other forms of heredity. I am not unaware 

 of Dr. Wallace's views on spirit hierarchies. I considered them 

 in my "Grammar of Science," and still hold them thoroughly 

 illogical and unscientific. 



(2) What I asked Prof. Cockerell to do was to explain why 

 the intensity in inheritance of mental and physical qualities 

 came out the satne. He may have views why they ought 

 to be different, but it remains for him to explain why soul 

 + heredity -I- environment in one case = heredity + environ- 

 ment in the other. 



(3) I believe the mental characters in man are far more 

 persistent than Prof Cockerell credits them with being. The 

 relations between head-measurements and intelligence are almost 

 identical whether we deduce them from young children or under- 

 graduates, and there is no apparent change of correlation when 

 we compare brothers at close and at more distant ages. It is 

 perfectly possible to determine from our data the proportions of 

 children at each age with given mental characters. Prof 

 Cockerell belongs to those critics who live in the region of 

 "may-be." If he will collect observations on some 5000 to 

 6000 children as we have done, he may still come down from 

 the region of " may-be " and be able to place fact against fact. 



University College, London. K.\rl Pearson. 



The Colours of Wings in Butterflies. 



"i'ouR correspondent in India, W. G. B. (Nature, February 

 13, p. 344), has been examining a butterfly in some ways like the 

 Morpho Cypris of South America, the difference being that the 

 latter has the upper side brilliant and the lower side brgwn. 



The Morpho can be placed so that the two wings on one side 

 of the body are metallic blue, while the other two are black ; with 

 a slight turn the two sides reverse colours. This seems to be 

 like the case of the Purple Emperor, in which all edges of the 

 scales facing one way are blue, and other edges are brown. A 

 ploughed field with furrows running east and west might after 

 snow and sunshine appear white from the north and black from 

 the south. In London it is easy to see the Morpho ; instead of 

 the furrows it is possible to take for illustration a common form 

 of advertisement. 



The Morpho, like the Indian specimen, presents shades ot 

 ultramarine, peacock-blue, and sea-green ; also in transmitted 

 light the scales are golden. In most cases of coloured surfaces 

 we are not yet able to point out the action on the light waves. 

 Prof Tyndall showed how small particles in air or water might 

 reflect blue waves and allow the larger red waves to curl round 

 them and go forward ; but this does not apply to a surface which 

 reflects the larger waves. It can only be said that coloured surfaces 

 are such as have the power at a minute depth of selecting some 

 waves for reflection ; in the case of gold leaf or some butterflies' 

 wings, the remainder of the light may be seen, transmitted almost 

 without any loss by absorption, as the thickness traversed is so 



NO. 1687, VOL. 65] 



