160 



KNOWLl'DGH 



Ai'KiL. I9i; 



and tlu- bahincc of rcsislnncc changes according as the 

 ti'Mipcratiiif of llio wire of one of tin- bulbs alters. 'l"hc 

 method appears aceiirate, but is of very limited application. 



Ne.xt. the MacLeod nange should be mentioned — most low 

 pressure measurements are made by means of such an 

 apparatus: but the values obtained are only reliable under 

 certain eircinnstances. This apparatus consists of a large 

 bulb with a small tube connected to it. Gas in the large bulb 

 can be pushed up into the small tube and measured imder 

 atmospheric pressure — from the change in volume from the 

 large volume of the bulb to the small volume of the tube, the 

 change in pressure from some low value to be found, to 

 atmospheric pressure can be obtained from Boyle's Law " that 

 the volume of a gas varies inversely as the pressure upon it. 

 if the temperature is constant." However, the reliability of 

 Boyle's Law at very low pressures is not absolutely certain, 

 though experiments do tend to show that at low pressures the 

 laws about gases still hold — and, further, the mercury itself has 

 a certain vapour pressure which is entirely unmcasurable and 

 neglected by this instrument. The walls of the tubes also 

 are apt to condense vapours in their pores. Measurements 

 of the pressure within X-ray tubes and electric lamps are 

 made by means of the MacLeod gauge — the gauge gives an 

 idea of the degree of vacuum attained, but is unreliable for 

 absolute measurements of pressure. 



The subject of vapour pressure is intimately connected with 

 that of smell. The nasal nerves are capable of detecting a 

 very small quantity of a substance — it is possible that one 

 molecnle of a substance, if it gets properly into contact 

 with the sensitive portion of the nasal organ, might affect it 

 perceptibly. The quantity of a gas. such as bromine, which 

 is detectable is very small indeed, and the pressures of vapours 

 detectable by smell are smaller than can be directly measured. 

 The effect is probably of a chemical nature — a loose combina- 

 tion, perhaps, is formed between the molecules of the vapour 

 and the substances of the nasal organ — it is noticeable that 

 the smells of all oxidising substances, when dilute, are practically 

 identical, which seems to show that the effect produced by 

 them on the nasal organ is similar. The sensitiveness of the 

 nasal organs of animals is well established, and it is indeed 

 marvellous to think that sufficient molecules which can be 

 smelt are left behind by a person who has passed a locality 

 perhaps an hour previous to the arrival of his dog ! 



There is much interesting work to be done in this direction, 

 and I have thought it worth while to draw attention to it in 

 this month's notes. 



SEISMOLOGY. 



By Charles Davison, Sc.D., F.G.S. 



THE ORIGIN OF EARTHQUAKES.— In a lecture on 

 "The elastic-rebound theory of earthquakes" (Univ. of 

 California Publications, Bull, of the Department of 

 Geology, volume VI., pages 413-4441. Professor H. I". Keid 

 discusses a theory of the origin of earthquakes which has 

 long been known, and I believe generally accepted, in this 

 country. The theory is that earthtjuakes are due to the 

 friction caused by the sudden sliding of the rock-masses 

 adjoining a fault-surface, that they are, therefore, merely 

 passing incidents of secondary importance in the growth of 

 faults. Professor Reid bases his account of the theory on 

 the remarkable displacements of the crust along the San 

 Andreas fault which gave rise to the Californian earth<|uake of 

 1906. He sums up the theory in the following terms: "1. The 

 fracture of the rock, which causes a tectonic earthijuake, is the 

 result of elastic strains, greater than the strength of the rock 

 can withstand, produced by the relative displacements of neigh- 

 bouring portions of the earth's crust. 2. These relative dis- 

 placements are not produced suddenly at the time of the 

 fracture, but attain their maximum amounts gradually during 

 a more or less long period of time. 3. The only mass move- 

 ments that occur at the time of the earthcpiake arc the sudden 

 elastic rebounds of the sides of the fracture towards positions 

 of no elastic strain ; and these movements extend to distances 

 of only a few miles from the fracture. 4. The earthquake 

 vibrations originate in the surface of fracture ; the surface 



from which they start has at first a very small area, which 

 may (piickly become very large, but at a rate not greater than 

 the velocity of compressional elastic waves in the rock. 5. The 

 energy liberated at the time of an earthquake was, immediately 

 before the rupture, in the form of energy of elastic strain of 

 the rock." 



ZOOLOGY. 



By Pkoi-kssor J. Arthur Thomson, M.A. 



AOUATIC VERTEBRATES OF SAHARA.— It seems 

 strange to speak of the freshw.iter fauna of the great desert, 

 but it is a reality and of very considerable geographical 

 interest. Besides a crocodile {Crocodilus itiloticns) and a 

 turtle, there are eight Amphibians and ten fishes. The 

 interesting point is this, as J. Pellegrin points out. that 

 while the more northern part of Africa has a Palaearctic 

 Mediterranean fauna, the aquatic vertebrates of the Sahara 

 are distinctively Ethiopian. 



ORIGIN OF GERM-CELLS IN SILK-MOTH.— Another 

 instance of the early segregation of the future germ-cells has 

 been found by Vaney and Conte in the development of the 

 egg of the silk-moth. At an early stage, when the blastoderm 

 has been formed and the germinal disc marked off, two large 

 cells appear between the blastoderm and the yolk which seem 

 to form the future germ-cells. The segregation is not so 

 precocious as in some other insects (the harlequin fly, 

 Cecidomyia, and Chrysomelid beetles), but it is nevertheless 

 a good instance of the early separation of the germ -plasm 

 from any share in body-making. 



PEARL MAKING. — A. Rubbel has made a very careful 

 study of pearl formation in the freshwater nms!n;\. Margaritana 

 iiiargaritifcra. and is against the theory- that they are the 

 sepulchres of parasites. He finds that they arise around 

 particles of a yellow substance, which resembles periostracum. 

 They originate in closed, single-layered sacs of epithelium, 

 which are constricted off from the external epithelium of the 

 mantle, and. like the mantle, are able to secrete all the layers 

 of the shell. The pearls grow by the deposition of layer after 

 layer on their surface. The coalescence of several pearl sacs 

 leads to the formation of curious pearl conglomerates. What 

 are called shell pearls are formed, to begin with, in the mantle, 

 and become secondarily attached to the shell. They are to 

 be distinguished from shell concretions, which are due to 

 intruded foreign bodies and show no concentric layering. 

 Besides rejecting the parasitic theory of pearl formation, 

 Rubbel calls attention to an overlooked fourth layer in the 

 shell. It is a clear intermediate layer, dividing the nacreous 

 layer into an outer and an inner stratum ; it is particularly 

 clear at the muscle insertions, but it occurs in other parts of 

 the shell and in the pearls, 



THE TURTLE'S EGG-LAVING.— S. O. Mast gives an 

 interesting account of the behaviour of the Loggerhead Turtle 

 in depositing its eggs. He watched the process on Logger- 

 head Key, Florida, in the month of July. The animal came 

 straight out of the water about sgven o'clock in the evening 

 and proceeded directly up the beach for fifty or sixty feet. 

 There seemed to be no selection of a place for the nest. By 

 moving the posterior part of the body from side to side, and 

 by throwing out the sand sideways and forward alternately 

 with the hind flippers, she dug a trench four feet long and 

 nearly ten inches deep in the middle. In this trench in a 

 very complicated way she made a cylindrical hole nearly as 

 deep as the length of the hind flippers. Into this hole the 

 eggs were dropped — perhaps a hundred altogether. Noise 

 and gentle touching did not cause any interruption. .After 

 laying the eggs the turtle covered them up. filling the trench 

 as well as the hole. " This completed, she returned to the 

 sea and entered only a few feet from the spot where she came 

 out. On the way down the beach I stood on her back and she 

 carried me (165 lbs.) apparently with but little effort." The 

 turtle under observation was out of water forty-two minutes, 

 spending three in reaching the nesting- place, four in making 

 the trench, eight in digging the hole, twelve in laying the eggs, 

 and fifteen in filling up, smoothing over and getting back to sea. 

 The rate of locomotion on land is about half a mile an hour. 



