April 21, 1893.] 



SCIENCE. 



219 



the same temperature have the same number of molecules to the 

 cubic centimetre, this shows that it is not the number but the 

 kind of molecules which determines the scattering. But perhaps 

 the most important experiments were those in which the dis- 

 charge was allowed to pass into another tube which had been 

 exhausted so far as possible. It was argued that if the cathode 

 discharge was due to the projection of atoms from the cathode 

 that it could not take place in an absolute vacuum. The tube 

 into which the discharge was to pass was exhausted as far as 

 possible, i e., until a twenty-centimetre spark would not pass 

 from one electrode of the absolute vacuum tube to the other. 

 Notwithstanding this extreme exhaustion, the discharge passed 

 freely through, as was shown by the phosphorescence of sub- 

 stances placed at the other end. The conclusion which Dr. Lenard 

 draws from this experiment is that the cathode rays are really 

 processes in the ether, and not due to the movement of atoms. 



On account of the difficulty of obtaining an absolute vacuum, 

 Dr. Lenard's results cannot be accepted as final. Even at the 

 exhaustion obtained by him it may be calculated that there are 

 quite a sufficient number of atoms left to produce the phenome- 

 non (using the results of J. J. Thomson and Chattock in the calcu- 

 lation), even neglecting the number contained in the layer of air 

 ou the sides of the tube, and which would be driven off into the 

 tube so soon as the discharge began to pass. Moi-eover, it is 

 quite possible to conceive that a discharge of atoms from the 

 cathode, on reaching a thin metal sheet, and being abruptly 

 stopped by it, might propagate an electric disturbance proceeding 

 from the other side of the sheet of metal, and so drive off another 

 set of charged atoms. If there were any way of obtaining an 

 absolute vacuum, of course the question could be answered defi- 

 nitely, but this is impossible, and we must wait for further results 

 before attempting an explanation. E. A. F. 



LETTERS TO THE EDITOR. 



#** Correspondents are requested to be as brief as possible. The writer's name 

 is in all cases required as proof of good faith. 



Chi request in advance, one hundred copies of the number containing his 

 communication will be furnished free to any correspondent. 



Tlie editor will be glad to publish any queries consonant with the character 

 of the journal. 



Low Temperatures. 



In your issue of Jan. 37, page 50, it is stated that the Franklin 

 Search Expedition, under Lieutenant Frederick Schwatka, in 

 1879-80, experienced a temperature of — 71° C. 



This is an error, as I have heard Lieutenant Schwatka in many 

 conversations refer to it as "seventy-one degrees below zero, 

 Fahrenheit." 



I enclose a copy of a letter now in a collection belonging to 

 my brother: — 



Tacoma, Wash., Sept. 15, 1892. 



On the third of January, 1880, my Arctic exploring party en- 

 countered a degree of cold of seventy-one below zero, Fahrenheit, 

 or one hundred and three degrees below the freezing-point of that 

 scale, the coldest we noted on the trip, and the coldest ever en- 

 countered by white men travelling in the field, for that day we 

 moved our camp some twelve miles. It will be a cold day when 

 that record gets left. Frederick Schw^atka. 



Fred. G. Pltjmmer. 



Tacoma, Wash., Feb. 11., 1893. 



Where is the Litre? 



It must be a source of regret to all interested in metrology that 

 so much time was expended in the preparation, and so much 

 space in the publication of the leading article in Science for March 

 17, entitled " Where is the Litre ?" etc. Even if the instruction 

 contained in the article be reinforced by the amusement which it 

 furnishes, the result is quite incommensurate with the labor 

 which must have been involved in its production. 



Ignorance of the recognized principles of metrology has led to 

 certain conclusions which will generally be harmless on account 

 of the very magnitude of their errors. The sermonizing finish to 

 the article, beginning with the sentence, "In spite of the much 

 lauded simplicity of metric measures," etc., may, however, mis- 



lead a few readers whose ideas have been befogged by the perusal 

 of the previous three pages. It will be well to remind them, 

 therefore, that the apparent bewildering confusion as to the value 

 of the litre has no relation whatever to the " simplicity of the 

 metric system." Indeed, the confusion might have been rendered 

 vastly greater, the alleged case against the metric system much 

 sti'onger, and the entire article more picturesque, if the author 

 had introduced the "gus" of Arabia, the "pik" of Egypt, and 

 the -'sun " of Japan, the value of each of which in metres must 

 always be a matter of considerable uncertainty. 



The following simple statements may be of value. It is gen- 

 erally agreed among metrologists that natural standards of length 

 and mass are not at present easily attainable. Our knowledge of 

 physical or astronomical constants must continually increase in 

 precision as methods and instruments are improved. Such con- 

 stants are, therefore, unsuitable for standards, because standards 

 should, first of all, be invariable as far as possible. Artificial 

 standards can be made of more convenient dimensions, can be 

 multiplied with almost any required degree of precision , and their 

 invariability is perhaps as well assured as that of any suggestive 

 national standard. 



It was originally jaroposed to derive the metre from the dimen- 

 sions of the earth. We know that the metre is not the one ten- 

 millionth of the quadrant of the meridian passing through Paris, 

 but that fact does not in the slightest degree lessen the value of 

 the metre as a unit of length. Its value is so nearly that, that it 

 is exceedingly convenient to use in ordinary calculations relating 

 to the earth, not requiring a high degree of precision. 



It was also proposed originally to establish some sort of a simple 

 relation between the unit of length and the unit of mass. As 

 length and mass have no natural relation to each other, any 

 numerical ratio must depend on a physical constant, namely, the 

 density of some selected kind of matter. The determination of 

 this must be a matter of experiment, and its value can never be 

 absolutely known. For this reason any relation between the 

 unit of length and the unit of mass must always be an approxi- 

 mation. The unit of mass must, therefore, be an artificial, inde- 

 pendent unit. 



The new international prototype of the metre is, in length, an 

 exact reproduction of the old metre of the archives, as far as can 

 be determined by the most recent and most perfect means of 

 comparison. The new international prototype kilogramme is 

 identical, in mass, with the old kilogramme of the archives, as 

 far as can be determined by the most precise and delicate weigh- 

 ings ever made. 



It was originally intended that the mass of the kilogramme of 

 the archives should be that of a cubic decimetre of pure water at 

 its maximum density. As this involves the knowledge of a physi- 

 cal constant, it was not possible to realize this relation exactly, 

 and it never will be possible. 



In determining volumes which do not exceed a certain limit, it 

 has been found that greater accuracy can ordinarily be secured 

 by the indirect method of determining the mass of a liquid of 

 known density, than by direct geometrical processes. The appli- 

 cation of the latter requires simple forms whose linear dimensions 

 may be easily and accurately measured. The former depends 

 only on the accuracy attainable in mass measurement and density 

 determination. 



This method of volume measurement has usually been regarded, 

 however, as a matter of convenience only. Thus, the U. S. gal- 

 lon is defined as a volume of 231 cubic inches; in standardizing 

 measures of capacity in gallons, it has always been customary 

 to use the indirect mass-density method. The mass of water 

 which has been assumed to represent this volume has varied from 

 time to time as our knowledge of the physical constants involved 

 advanced. 



The litre was originally assumed to be identical in volume 

 with the cubic decimetre, and there could be no possible objection 

 to confining the term litre strictly to this meaning. But, as noted 

 above, it being vastly more convenient to use the mass-density 

 method in determining volumes, much of the uncertainty of pre- 

 cise volumetric work would be avoided by defining the litre as the 

 volume of a kilogramme of water at maximum density. 



