July, 1906.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



483 



pressure was accompanied by a corresponding rapid in- 

 crease in the thickness. On subjecting the first black film 

 to sudden increases in pressure, it was possible to convert 

 it quickly into a second black ; if then the pressure was 

 ^uddenly diminished, the second black became a first black. 

 In many cases it was possible to continue this thickening 

 until the film showed the yellow of the first order. These 

 changes are attributed to the heating (or cooling) produced 

 by adiabatic compression (or expansion). The heating pro- 

 duced by compression is accompanied by an increased 

 vapour pressure, and consequently evaporation takes place 

 from the surface of the film ; while on the other hand 

 adiabatic expansion causes cooling and condensation on the 

 film. Measurements of surface tension show that there 

 is no discontinuity in the surface tension corresponding to 

 the discontinuity in the thickness at the junction between 

 the two black films. When the second black is obtained 

 by slightly increasing the pressure, it appears as small, 

 perfectly round, black spots in the first black film, which 

 expand to replace the first black as the pressure is increased. 

 If then, the pressure be slightly lessened, perfectly round 

 spots of the first black will appear in the film of the second 

 black. 



The solutioti was made in the following manner : — 7 

 grams of oleic acid (relined) and 2 grams of caustic soda 

 are thoroughly boiled in about 100 c.c. of distilled water. 

 This gives about 7.5 grams of oleate of soda, which is 

 diluted to about 50 of water to i of soda, and again boiled. 

 An excess of acid is not harmful, while an excess of soda is 

 decidedly so. 



The Effect of Sound on Water Jets. 



A problem which has been the subject nf much discussion 

 is the external reality (or otherwise) of difference-tones. 

 These notes have a pitch which is the difference of the 

 pitches of two sources of sound, which are acting together, 

 and the question in dispute is whether these are produced 

 externally to the ear or are due to the mechanism of the 

 ear itself. Some experiments bearing upon this question have 

 recently been made by Mr. Belas, as a de\-elopment of experi- 

 ments of Professor VV. F. Barrett. When a jet of water falls 

 upon a membrane while a tuning fork is sounding in contact 

 with the support of the jet, it excites a sound of the same 

 pitch in the membrane, owing to the jet being broken 

 into drops at intervals corresponding to the frequency of 

 the fork. When two forks of pitches 384 and 512 vibra- 

 tions per second influence the jet, the note given by the 

 membrane has a pitch of 128, that is, two octaves Ijelow 

 the higher fork. This effect is produced even when the 

 notes of the tuning forks themselves are nearly inaudible. 

 When instantaneous photographs of the jet are obtained, 

 with each, and with both the forks sounding, the spacing 

 of the drops corresponds with the separate pitches and with 

 the difference-tones respectively. Now, the peculiarity of 

 this experiment consists in that the difference-tone is heard 

 even though the generating tones have disajjpeared. This 

 then, seems to be a case in which the explanation given by 

 Helmholtz cannot possibly be true. Helmholt/: considered 

 that they are in all cases produced only when the generating 

 tones are intense, and are indeed due to the failure of the 

 law of simple proportionality between force and displace- 

 ment. The explanation given by the authors is more in 

 accordance with the original explanation of Dr. Young. 

 When two notes are of dil'ferent pitch there are moments at 

 equal intervals at which the resultant amplitude of vibration 

 is greatest, i.e., whenever both notes are in the same 

 phase. The number of such events per second is equal 

 to the frequency of the difference-lone ; and it seems 

 possible that the ear picks out these rapidly recurring, in- 

 tense sounds, neglecting those which are feeble in com- 

 parison ; and, if they recur sufficiently rapidly, interprets 

 them as a musical note. An analogy of the action has been 

 given by Professor Poynting. " .Suppo.sc a reservoir is at 

 the sea-shore, just below the level of the highest spring- 

 tide, and with some contrivance for emptying it slowly. 

 Then it will fill once a fortnijjht, and its rise and fall 

 will have a frequency equal to the difference in frequencies 

 of the solar and lunar tides. Its oscillations will, of course, 

 not be harmonic, but its chief vibration will be fortnightly." 



ZOOLOGICAL. 



By R. Lydekker. 

 Butterfly Swarms 



The April number of Spolia Zcylanica (of which the Editor 

 has received a copy) contains an interesting account, by Mr. 

 E. E. (ireen, of the enormous swarms of white and yellow 

 butlerfiies which make their appearance every spring and 

 autumn in Ceylon. In a space bounded by two points 30 

 feet apart and 12 feet from the ground it was calculated 

 that about 150 butterflies passed, on an average, every 

 minute. From this it was estimated that the number of 

 butterflies passing in one direction between two points a 

 mile apart and 12 feet from the ground would be 26,400 per 

 minute, 1,580,000 per hour, and 6,336,000 between the hours 

 of II a.m. and 3 p.m. on one day alone. 



The Cairo " Zoo." 



F'rom the report for last year (of which a copy was sent to 

 the Editor) it is satisfactory to learn that the Zoological 

 Gardens at Giza, near Cairo, under the superintendence of 

 Captain Stanley Flower, continue to make satisfactory pro- 

 gress, despite some losses from unfavourable weather. 



Okapi for the British Museum 



Captain Gosling, now on the Congo frontier, has already 

 sent to the Natural History Branch of the British Museum 

 a complete skin of the Nigerian giraffe, as well as speci- 

 mens of many other mammals, and we now learn that he 

 and ills companion. Captain Alexander, have succeeded in 

 procuring an okapi skin for the national collection. These 

 two explorers seem to have come into very close contact with 

 the animal, and have furnished some important information 

 with regard to its habits ; but the actual trapping was ac- 

 complished by a Portuguese collector accompanying the 

 expedition. 



Some New Mammals. 



Mr. O. Thom:is, of the- British .Museum, has recently been 

 enabled to describe quite a number of new genera of small 

 mammals. .Among the more recent additions to the list is 

 a mouse from Persia, for which the name Calomyscus has 

 been proposed ; this species being of special interest on ac- 

 count of the fact that it is totally unlike all other Old World 

 mice and near akin to North American types, more especi- 

 ally Fci(/myscus. A second novelty is a new genus (I'laty- 

 iiwi)s) of free-tailed bats from North-East Africa, this bat 

 being distinguished from its relatives, the mastitT-bats 

 (Molussus), by the absence of wrinkles in the lips and the 

 curiously flattened head. Here, too, may be appropriately 

 noticed the description by Mr. R. B. Kinnear of a new race 

 of British field-mouse (Mus sytvaticus fridaviensis) from Fair 

 Island, south of the Shetlands. 



Papers R.ead. 



.At the meeting of the Zoological Society held on May i, 

 the Hon. Walter Rothschild read a supplemental paper on 

 gorillas and chimpanzis, at the same time exhibiting a 

 splendid series of specimens mounted for his museum at 

 Tring. All the gorillas were referred to a single species, 

 with several sub-species, the typical race being represented 

 by a dark-headed and a red-headed phase. Australian 

 mammals formed the subject of a paper by Mr. O. Thomas, 

 while Mr. F. E. Beddard contributed information '.\ith re- 

 gard to the anatomy of snakes. Mr. H. J. El-.ves, Sir 

 George Hampson, and Mr. J. H. Durrant severally read 

 papers on the butterflies and moths collected during the 

 recent expedition to Tibet. M the meeting of the same body 

 on May 15 Mr. Thomas read a further paper on .Australian 

 mammals, while Mr. R. I. Pocock discussed the breeding 

 habits of certain monkeys in the Society's menagerie. On 

 the same occasion Mr. J. N. Halbert described water-mites 

 from Lake Nyasa, Messrs. Benham and Dunbar contiibuted 

 notes on the skull of a ribband fish, Dr. von Lii.stow dis- 

 cussed hair-worms from Korea, while Mr. G. .A. Boulenger 

 conununicated an account of certain new reptiles from 

 Uganda. 



