52 



SCIENCE. 



[Vol. XX. No. 494 



II. In any one metal the force of cohesion varies inversely as 

 the square of the distance between the centres of its atoms. 



We may expect these facts to be of great use in the study of 

 the properties of matter. For, knowing the size and weight of 

 the atoms and the velocity with which they move, all that was 

 wanting to enable us to calculate the behavior of the atoms of 

 matter, in the same way as we do the motions of the planets, was 

 a knowledge of the laws of the force wliich holds them together; 

 and, from the evidence given above, I have no doubt that you 

 will agree with me in saying that we have at least made a begin- 

 ning in that direction. 



A few words might be said about Poisson's ratio. It is, as I 

 said, not fair to argue from the behavior of cork or india rubber 

 that there is no relation between longitudinal extension and 

 lateral contraction, or between a and b. When we compress a 

 cork we are not compressing the substance which forms the cork 

 any more than we are compressing a piece of paper when we 

 crumple it up in our hand. A cork is like a dry sponge, and 

 when we squeeze a sponge up in our hand we are simply doubling 

 lip the cell- walls, not compressing the substance of the sponge. 

 Tlieonly way in which we can determine the compressibility of 

 cork is to soak it in ether or some substance whicli fills all its 

 pores and then subject it to hydrostatic pressure. In the same 

 way when we stretch india rubber, or ivory or jelly, the longitudi- 

 nal extension of the piece of rubber is not in the least a measure 

 of the longitudinal extension of the substance of the rubber. All 

 such substances are made up of two parts; rubber, for instance, of 

 a hard elastic skeleton, insoluble in most solvents, and of a soft 

 plastic substance, soluble in many solvents, by use of which the 

 two parts may easily be separated, similarly ivory and jelly. Let 

 us take a square cell as in Fig. 3, the walls of which are of elastic 

 material and the contents an incompressible plastic substance. 

 Suppose it to be extended till its length is 4 centimeters and its 

 breadth and thickness each 2 centimeters, as in Fig. 4. The total 

 area of cell-wall is 40 square centimeters, and the total volume of 

 incompressible contents i§ 16 cubic centimeters. Imagine the cell 

 to be released, it will regain its position as in Fig. 3, and form a 

 cube of side 3.53 centimeters. In this case, the volume being the 

 same, the cell area will be 38.1 square centimeters. So we find 

 that by stretching the cell till its length was 60 per cent greater 

 than before, we have only had to stretch the cell-walls 5 per 

 cent. This gives us the explanation of the well-known fact that 

 stretched rubber contracts %vhen heated. For if we heat the cell 

 shown in Fig. 4 the incompressible contents will expand and tend 

 to make the cell-walls take that shape in which they can hold the 

 most. This is obviously that of the original cube, therefore the 

 result will be a contraction. 



Of course the formulae, derived from this theory of cohesion, 

 give us the means of calculating the physical properties of metals 

 which have never been examined, or even discovered. For ex- 

 ample, it shows us that we have at our disposal a metal far 

 superior to any metal yet known, one which is stronger than iron, - 

 lighter than aluminium, and a better electrical conductor than 

 silver. Aluminium, in spite of its lightness, is too weak mechani- 

 cally and too poor a conductor to be used in many cases. But 

 this new metal is four times as strong as aluminium, and is twice 

 as good a conductor of electricity. The metal referred to is 

 glucinum or beryllium. All that is known about it is that it has 

 an atomic weight of 9.1 and a density of 1.7 to 2, the exact figures 

 not being known. But from these scanty data we can deduce 

 the following figures: 



Metal Rigidity Tensile st'gth Conductivity Sp. gr. 



Alumin. 350 x 10» 18 Kgms 50 3.75 



Silver 280 37 100 10.5 



Iron 750 43-65 14 8 



Calculated for ..„„„ „^ ,.„ 



Glucinum ^^^^ '^^ ^O'^ 3 



We also see why diamond is so hard, and that there is only one 

 other thing that might possibly scratch it, and that is a crystal of 

 manganese. With the exception of glucinum, none of the other 

 metals, eitlier discovered or to be discovered, are likely to be any 

 better than those we have now. 



NOTES ON LOCAL HEMIPTERA-HETEROPTERA. 



BY E. B. SOUTHWIOK, PH.D. 



In the COBISIDjSI Corisa Harrisii Uhl. is very common 

 in our park lakes, and the drag-net bring'S many of them to 

 land at every haul. Another species as yet undetermined is 

 about one-third the size of Harrisii, and equally abundant. 



In NOTONECTID^ Notonecta undulata Say. is very 

 common. This was at one time known as variabilis Fieb., 

 a name quite appropriate, for they are variable to a marked 

 degree, some of them being- nearly white, while others are 

 very dark. Notonecta irrorata Uhl. is also common, and 

 is a very beautiful insect, and more uniform in coloration. 



In NEPID^E Ranatra fusca Pal. Beauv. is our only 

 representative, as far as my observation goes; this was at one 

 time known as R. nigra H. Schf. 



In BELOSTOMATID^ we have two species. Benacus 

 griseus Say., that g-iant among- Hemiptera. This much-named 

 creature has been known as B. haldemanus Leidy, B. har- 

 pax Stal., B. ruficeps var. Duf., B. distinctum Duf., and 

 B. augustatmn Guer. ; but at last has settled down to B. 

 griseus, which name, I hope, gives credit where it belongs. 

 Zaitha flimiinea Say. is very common in our lakes, and the 

 females are often taken with their backs completely covered 

 with eggs, deposited in regular rows upon the elytra; at the 

 same time the young of all sizes will be brought up with the 

 drag-net. 



In the family HYDRODEOMICA and sub-family Sal- 

 diDjE I have but one representative species, Salda orhiculata 

 Uhl., and it is exceedingly rare. 



In the subfamily HYDEOBATID.E I have taken three species, 

 viz., Limnoporus rufoscutellus Lat., Limnotrechus mar- 

 ginatus Say. , and Hygrotrechus remigis Say ; they are all 

 about equally common on the waters of our lakes and in 

 ditches and pools. 



In the family REDUVID^ the sub-family Piratina is 

 represented by Melanolestes picipes H. Schf., which is quite 

 common under stones along with Carabidoe. 



In the sub-family Reduviina we have three species. Diplo- 

 due luridus Stal. is very common with us, but in Professor 

 Uhler's list it is only given as from Mexico. Acholla mul- 

 tispinosa is also common; this has beeu known as A. sex- 

 spinosus WolfiF., and A. subarmatus H. Schf. 



Sinea diadema Fabr. is not rare with us; this insect has 

 had a number of names, and has been studied as S. multi- 

 spinosus De G., S. hispidus Thunb., and S. raptatoriiis 

 Say. I have a pair of insects from this State labelled Har- 

 pactor cinctus Fabr., which are probably what is now known 

 as Milyas cinctus Fab. They are of a beautiful pinkish- 

 white color, and have the limbs banded with black. 



In the sub-family Corisina three species of Corisciis are 

 represented. Coriscus subcoleoptratus Kirby, a very com- 

 mon and curious insect, and formerly known as C. canaden- 

 sis Pro v., C. annulatus Reut, which is very rare, and C. 

 ferus Linn, rather common. 



In the family PHYMATIDyE the sub-family Phymatina 

 is represented by that very common and curious insect Phy- 

 mata Wolffii Stal. Phymata erosa, which is quoted as com- . 

 mon throughout the State of New Jersey, I have never found 

 here. 



In the family TINGITID^ and sub-family Tingitina I 

 have Corythuca arquata Say. as one of the most common. 

 This species of Tingis is found on the butternut, and was at 

 one time known as Tingis juglandis Fitch, and Dr. Riley 

 found it on the white oak. 



