March 17, 1893.] 



SCIENCE. 



145 



every one of our birds been deemed of any great importance by 

 ornithologists. Of late, owing to the discovery of numerous 

 errors that had crept into our nomenclature, careful attention has 

 been paid to a species from the time of its advent into the world 

 to a period when beyond all doubt it has reached its maturity. 

 To the collector who accumulates a series, it is only too apparent 

 how great is the difference between individuals, and that his 

 series is not complete until each and every phase of plumage 

 from various widely separated localities is represented. 



Late in the season, while the full migration is at its height, a 

 bird is secured which for the life of him he cannot name; in vain 

 he searches the literature, compares specimens, and puzzles and 

 worries only to find it at last an old acquaintance flitting under 

 new colors. I have in mind a young man who, although not an 

 accomplished ornithologist, ought to have known better, and 

 who essayed to publish a list of the birds of the locality in which 

 he lived. One winter he secured a bird entirely unknown to him, 

 and in his dilemma sent it to the Smithsonian for identification; 

 on its return the label hore: "American Goldfinch in winter 

 plumage." This may tie a little foreign to the subject but it 

 shows how necessary was a thorough knowledge of the life-history 

 of the species. Nor was it so very long ago that the "Gray 

 Eagle,'" which for years was accorded speciSc rank, was found 

 to be but an immature phase of Haliceetus leucocephalus, while 

 Oidemia perspioillata troivbridgii was shown to be but a seasocal 

 variation of perspicillata proper. Even tothisday it appears not 

 to be generally known that the Golden Eagle takes from three to 

 five years to acquire its full plumage; that the Bald Eagle attains 

 his highest plumage at the a^e of three, the various intermediate 

 stages being known as the Black Eagle, Gray Eagle, etc., and 

 that the Little Blue Heron is pure white the first year, mottled 

 and variegated with blue in every conceivable manner the second, 

 and attains the perfection of its plumage only at the age of three; 

 yet such are the facts. These are hut isolated cases, while any 

 day may bring about the unification of some two forms which 

 at present are considered at least sub-specifically distinct. 



BOILING-POINT AND RADIUS OF MOLECULAR FORCE. 



BY T. PBOCTOR HALL, CLARK UNIVERSITT, WORCESTER, MASS. 



When a bubble of its own vapor exists in a liquid the pressure, 

 P, upon it is the sum of the three pressures: — 



A, due to the air, 



W, due to the water above the bubble, 



C, due to molecular cohesion. 



Let us suppose, tor convenience, that the bubble is so close to 

 the surface that W may be neglected. When the radius, r, of 

 the bubble is large compared with i?, the radius of molecular 

 force (i. e., the distance at which a molecule ceases to exert a 

 a sensible attraction), the pressure, C, over its diametral plane is 

 equal to the surface tension, T, across the circumference. That 

 is to say, 



7rr= C= 2wrT 

 or C- 2T/r. 

 Ihen P = A + 2T/ r\ and the temperature must be such that Pis 

 balanced by the molecular energy of the vapor if the bubble is to 

 continue to exist. As r increases 2T/r decreases, and for bub- 

 bles of ordinary size may be neglected in comparison with A, the 

 ordinary pressure of the air. Hence the lowest possible boiling 

 point of a liquid is such that the vapor pressure is just sufficient 

 to overbalance the air pressure. But at one or more points in the 

 liquid the temperature must be very much higher, or no bubbles 

 of vapor could be formed. This condition occurs whenever a 

 liquid is boiled in a rough vessel. 



If a liquid be uniformly heated no bubbles can be formed until 

 the temperature is such that P = A + C for the whole liquid when 

 the bubbles are first formed. When this point is reached bub- 

 bles are formed everywhere, the pressure upon them decreases 

 very rapidly as they increase in size, and the liquid explodes. 

 The explosion point, like the boiling-point, depends in part upon 

 the pressure of the air, but has a definite lower limit when 

 A = 0. 



Unfortunately the value of C in terms of the siirface tension 



cannot be calculated directly for the explosion point; but a 

 probable value may be found as follows: — 



When a U-shaped wire in an inverted position is drawn up 

 from a liquid, in many cases a film is formed between the wire 

 and the liquid surface. For a pure liquid the thickness, k, of the 

 film is nearly constant, though it varies greatly in some solutions. 

 The film has a measurable tension, 2T, across every linear cen- 

 timetre on its surface. In other words, a force of 27' is drawing 

 apart, against the forces of cohesion, a liquid whose section is 

 (K X 1) square centimetre. It seems probable, therefore, that 

 the liquid will gi\e way at every point when theexpansive force 

 opposing C becomes 2T/k on each square centimetre; so that at 

 the explosion point „_ . .oji/i. 



In 1861, Dufour (Compte.s Rendus, f 2, p. 986) succeeded in heat 

 ing water to 175° C. and chloroform (which boils at 61°) to 98°, 

 under ordinary air pressure, without explosion. Assuming that 

 these are approximatt-l) the explosion points of water and chloro 

 form, we may calculate, from the known values of the surface 

 tensions and the vapor pressures at these temperatures, that the 

 value of k for water is 123 /z/i (1,000,000^^ = 1 mm.), and for 

 chloroform 200 ftft. From a solitary case, vvhich may be only a 

 coincidence, it would be rash to generalize; yet it is interesting 

 to notice that the ratio of these two values of k is almost exactly 

 the ratio of the molecular diameters of water and chloroform. 



Now R, the radius of molecular force, is known to lie some- 

 where between k and k/2 (see Jour. Chem. Soc, 1888, p. 222). 

 Hence, if the preceding equality of ratios be found to hold for 

 other liquids we shall have the theorem that the radius of mole- 

 cular force is proportional to the diameter of the molecules. 



Quincke (Pogg. Ann., 137, p. 402, 1869), by a method likely to 

 give results a little too low, measured R and found for water 

 54 /j.fi, a value which is in close accord with that given above. 



The experimental determination of the explosion points of dif- 

 ferent liquils requires no complicated apparatus and would have 

 considerable scientific interest. I make the suggestion for the 

 use of any one who has time and inclination for research without 

 the advantages of a well-equipped laboratory. 



DR. GEORGE VASEY. 



De. George Vasey. Botanist of the Department of Agriculture, 

 died, in the City of Washington, March 4, 1893. He was born on 

 Feb. 28, 1822, at Scarborough, Yorkshire, England, and came to 

 America when a child. He graduated from Berkshire Medical 

 College at Pittsfield. Mass., in 1848. and settled in Illinois, where 

 he practised his profession for twenty years. He was appointed 

 Botanist to the Department of Agriculture in April, 1872, and 

 held the position until his death. As Botanist to the Depart- 

 ment he was Honorary Curator of Botany in the U. S. National 

 Museum, and it is largely from his efforts that the present her- 

 barium of over 25.000 species has been accumulated and arranged. 

 His main work has been upon grasses, and among other papers he 

 has printed '• Descriptive Catalogue of Native Forest Trees of the 

 U. S.,'' 1876; "Grasses of the United States: A Synopsis of the 

 Tribes with Descriptions of the Genera," 1883; " Agricultural 

 Grasses of the United States," 1884; "Descriptive Catalogue of 

 the Grasses of the United States," 1885; "Report of Investiga- 

 tions of Grasses of the Arid Regions," two paits, 1886-87; 

 " Grasses of the South," 1887; "Agricultural Grasses and Forage 

 Plants of the United States," a revised edition, with 114 plates 

 of •' Agricultural Grasses," 1889; "Illustrations of North Ameri- 

 can Grasses; Vol. I.. Grasses of the Southwest," 100 plates with 

 descriptions, 1891 ; Vol. II., Part 1 of the same, " Grasses of the 

 Pacific Slope and Alaska," 1892; "Monograph of the Grasses of 

 the United States and British America" (Vol. III., No. 1, Contri- 

 butions from U. S. National Herbarium) 1892. 



He was a delegate from the Department of Agriculture and 

 the Smithsonian Institution to the Botanical Congress in Genoa, 

 last September, returning immediately after the adjournment of 

 the congress. He was a member of the Biological and Geographi- 

 cal Societies of Washington, and a Fellow of the American Asso- 

 ciation for the Advancement of Science. He was taken sick on 

 Feb. 2S, and died after a short illness on the morning of March 

 4. of constriction of the bowels. 



