April 6, 1893] 



NA TURE 



537 



Trans, xcvii. 249. Known quantities of the air to be tried, 



and of nitrous gas being mixed, were admitted into a 



graduated tube, which he [Priestley] denominated a eudio- 

 meter." This seems to point directly to Priestley as the author 

 of the name as he certainly was the author of the process, (It 

 may be mentioned in passing that, in this paper, Pepys describes 

 the method of calibrating eudiometers, by pouring in equal quan- 

 tities of mercury from a tube closed at one end and with the 

 mouth ground flat, against which a piece of plate glass is pressed 

 in order to obtain an exact measure of the mercury.) 



With these directions I searched in the library of the Royal 

 Society and found Magellan's book ; but he uses the name eudio- 

 meter as if it were well known. Mr. White, the librarian, very 

 kindly interested himself in the matter and found in Priestley's 

 book, "Observations on different kinds of Air," a statement 

 that he had received from Landriani one of his eudiometers 

 together with a description that he asks Priestley to print, but 

 the latter excuses himself on the ground that it would not be 

 convenient for him to publish the letter at that time. Mr. 

 White found the title of a book by Marsilio Landriani, 

 " Ricerche fisiche intorno alia salubrita dell' aria" (Milano, 

 1775, 8^). It is not in the libraries of the Royal or of the 

 Chemical Society, and the title does not appear in the catalogue 

 of the library of the Royal Institution, but last week I found 

 the book at the British Museum. On page viii. of the Intro- 

 duction there is a paragraph of which the following is a trans- 

 lation : "The account of the discovery of nitrous air and of 

 some of its principal properties is briefly set forth, certain 

 defects of Priestley's apparatus are removed, and there is 

 added a detailed description of the Eudiometer, for that is the 

 name which I give to my little instrument, from Eudios, a Greek 

 word signifying goodness of the air (bonta dell' aria) accom- 

 panied by the more useful precautions for its construction." 

 There are some plates at the end of the book containing draw- 

 ings of the apparatus, and one of them is marked " Eudiometro 

 1775." This seems to leave it without a doubt that it is to 

 Landriani that we owe the word. 



Next as to its exact meaning : by tradition we have been 

 taught that the eudiometer is an apparatus for measuring the 

 goodness of air, and this is obviously what was intended by 

 Landriani. The New English Dictionary derives it from 

 «i/5«oj clear (weather) and /xfTpov ; Roscoe and Schorlemmer 

 derive it from ei>5/a, fine weather, and fiirpov ; these meanings of 

 the Greek words are no doubt correct, and the name would 

 seem to be more applicable to some kind of weather glass, a 

 signification which the above quotation shows could hardly 

 have been in Landriani's mind. Herbert McLeod. 



Cooper's Hill, March 21. 



a mere logomachia. My purpose is to show that cave-animals 

 afiFord a pailicular case of the general problem how to reconcile 

 the law of recapitulation with the theory that adaptations or 

 degenerations are explained by the selection of congenital 

 variations. J. T. Cunningham. 



Blind Animals in Caves. 



Although in my previous letter I did not give evidence 

 directly supporting the proposition that blind cave-animals are 

 born or hatched with relatively well-developed eyes, that thesis 

 is a good deal more than a mere supposition, as Prof. Lankester 

 calls it. Nor did I, as Prof. Lankester asserts, proceed to state 

 that no such fact is known or recorded. The condition of the 

 eyes in the newly-born young of the viviparous Amblyopsis, or 

 other cave- fishes, does not appear to have been investigated, 

 although living young were born under observation as long ago 

 as 1844, and exhibited as spirit specimens to the Belfast Society 

 of Natural History. Nor have the early stages of the European 

 Proteus been obtained. But, on the other hand, with respect 

 to cave Crustacea, Tellkampf, the original describer of the blind 

 Cambarus pellucidus of the mammoth cave, stated that the eyes 

 were larger in the young than in the adult (A. S. Packard, 

 Amer. Nat, 1871), and Garman {^Bull. Mus. Comp. Zool xvii. 

 1888-89) states that in very young specimens of C. setosus, the 

 blind crayfish of the Missouri caves, "the eyes are more pro- 

 minent, and appear to lack but ihe pigment." In another 

 blind subterranean species, Troglocaris Schmidtii, occurring in 

 Central Europe, Dr. Gustav Joseph found and demonstrated 

 that the embryo in the egg was provided with eyes. (See 

 Packard, " Cave Fauna of N. America," Nat. Acad. Set. vol. iv. 

 Mem. I.) 



Thus, although it is obvious enough that further investigation 

 of the development of cave-animals is required, it cannot be 

 said that it is altogether a " hitherto unattempted embryoiogical 

 research." A discussion of this kind ought not, however, to be 



The Value of the Mechanical Equivalent of Heat. 



In Nature for March l6 you published a summary or 

 a communication which I had the honour to make to the 

 Royal Society. My conclusion as to the value of the C.G.S. 

 unit of heat was 41940X lo^ ergs (see Nature, p. 478), and I 

 added the following comment: "If we express Rowland's 

 result in terms of our thermal unit we exceed his value by i 

 part in 930, and we exceed the mean value of Joule's (selected) 



determinations by one part in 350 if we attach equal 



value to a// the results published by Joule his value exceeds ours 

 by I part in 4280. " 



I have received so many communications with regard to this 

 last statement, that you will perhaps permit me to answer my 

 correspondents through your columns. 



I thought it unnece>sary in a short summary to point out 

 that the value (in gravitation and Fahrenheit units) result- 

 ing from Joule's own experiments is not the usually ac- 

 cepted 772 'SS. To me it appears an extraordinary thing that 

 772 is to this day given in the text-books when, so far back as 

 i88o, Rowland conclusively proved that the results obtained 

 from Joule's experiments give a higher value (see Proceedings, 

 American Academy, March 1880). 



In 1879 Rowland forwarded to Joule a thermometer by 

 Baudin, which had been directly compared with Rowland's air 

 thermometer. Joule himself then made a careful compari.'ion 

 of his thermometer with the Baudin one, and communicated 

 the results to Rowland. The complete table is given on p. 39- 

 of the paper already referred to. In addition to the correction 

 thus shov\n to be necessary, further corrections for the capacity 

 for heat of the calorimeter and contents were included, and as 

 the results were published in Joule's lifetime, there can be little 

 doubt but that these corrections received his approval. 



I give an example (from p. 44) of Rowland's corrections : — 



Joule's value (by friction of water, in 1878) 



Correction for thermometer 



,, capacity for heat 



,, latitude (Baltimore) 



,, vacuum .. 



Corrected value at Baltimore 



7727 

 + 3-2 



-t- -2 



+ -9 

 - '9 



7761 



It is evident that Rowland did not claim for his air thermometry 

 an order of accuracy greater than ±01". In the appendix to 

 his paper (p. 197) he remarks that (/"a certain improvement (not 

 then adopted by him) was made, " it is probable that an accuracy 

 of 01° C. could be obtained from the mean of two or three 

 observations. I believe that my own thermometers scarcely 

 differ much more than that from the absolute scale at any point 

 up to 40° C." 



A study of Rowland's methods, and of the tables given in his 

 admirable paper, leads to the conclusion that it is possible that 

 his thermometry is in error by i in 1000 over the range 15° to 

 25°, and such an error would suffice to bring together the re- 

 sults (both in the value of J and in the temperature coefficient of 

 the specific heat of water) obtained by Rowland and myself. 

 The error would, however, but slightly affect the correction of 

 Joule's results. 



If we attach arbitrary values to Joule's later experiments, the 

 mean of the corrected values (by Rowland's thermometer) is 

 77675 {g = 32'I95) ; and the mean of all his determinations by 

 various methods is 779 17,' and we may regard the above a* 

 within 1 in locxa of the value resulting from Joule's own work 

 on this subject. 



I trust that in future our engineers and text-book writers will 

 (even if they ignore the work of later observers) do Joule the 

 justice of discarding the traditional 772, and adopt a value 

 more iti harmony with the investigations of that great ex- 

 perimentalist. E. H. Griffiths. 



12, Parkside, Cambridge. 



1 In terms of a thermal unit at 15° C. 



NO. 1223, VOL. 47] 



