246 



• KNOWLEDGE ♦ 



[April 27, 1883. 



other prolific creatures, from vertebrates to insects, and 

 ol)j(icts still lower. Tlirre arc two things to bo considered 

 in such cases. First, if from the number and power of 

 enemies, the clltHts of changing clin.ate, or any other 

 cause, a species is subject to great mortality, it requires a 

 corresponding fertility to keep it in Ijeing ; but when, for 

 example, a lish lays millions of eggs, we may consider that, 

 in the system of nature, eggs may be required as well as 

 fishes. A human contriver or workman would, if he 

 could, make a codfish outright, and not build up 

 an egg first out of millions of molecules, and then 

 hatch it into his fish. In nature, however, all inter- 

 mediate steps may be regarded as necessary parts of a 

 great plan, and the quantities of matter passing from the 

 unorganised to the organised condition as a tiux and reflux 

 dependant upon laws essential to the system as a whole. 



When we want live aphides to examine under the micro- 

 scope in a vigorous condition, we must handle them with 

 extreme gentleness, or their soft and delicate bodies will 

 be injured and the creature killed. Their slightness of 

 structure is, however, accompanied with great endurance 

 of conditions that would be quickly fatal to many stouter 

 organisms. Most insects would be rapidly killed by im- 

 mersion in paraffin oil ; but young and vigorous aphides will 

 often live for some time, and occasionally for hours in this 

 fluid, such as is burnt in lamps. If two or three of the insects 

 are very carefully placed in a little cork cell, mentioned in 

 the last paper, filled with paraffin oil, and covered with thin 

 glass, they are in a handy condition for examination. The 

 result of numerous experiments made with the best 

 American petroleum oil, commonly called crystal oil in the 

 lamp-shops, is that the survivals are very uncertain, but 

 sufficiently frequent for the process to be well worth 

 trying. They keep pretty quiet in the fluid, and it enables 

 higher power to be used with convenience. A half-inch 

 objective, magnifying about 100 linear, with a full-sized 

 instrument, is very handy. The illumination should be 

 varied ; but one of the best ways is to use both an 

 achromatic condenser and a lieberkuhn, or little silver 

 reflector, at the end of the objective. The largest hole 

 and central stop of the condenser will give a fine 

 dark ground illumination. When used in combination with 

 the lieberkuhn, it lights up the inside of the object, 

 while the less transparent parts receive reflected rays 

 from the silver surface. The student will find a great 

 many cases in which this mode of treating a refractive 

 and reflective object produces the best results. The eyes 

 of the aphis seen i:i this way are like half mulberries, and 

 the little eye projecting from the corner of the larger group 

 is well displayed. Where the view of the compound eyes 

 is a full -face one, the dark pigment is seen so strongly that 

 its true position is concealed. A profile view shows the 

 little lenses to be clear, like glass, and the pigment to be 

 behind them. A comparison of these eyes with those of 

 a common fly will show a considerable difl'erence. The 

 aphis lenses all look hemii?pheres, much like the projections 

 of a mulberry, or blackberry, while the common insect form 

 is hexangular — a shape produced by squeezing a number of 

 globes together. 



Many parts that looked plain with a lower power, 

 appear beautifully ornamented under a higher one. The 

 cornicles of the specimen before us are prettily imbricated, 

 and the antennie are very graceful objects. It is diflicult, 

 without dissection, to make out much of the internal 

 organs, but the observer will be struck with the quantity 

 of oily-looking globules. Indeed, between the incipient 

 young ones and the fat or oil, there seems little room for 

 anything else. This fat supply is probably connected with 

 the resistance of the little animals to cold, and may serve 



as a food reservoir in bad times. Just below the knee- 

 joint of the legs, at the top of the tibia, a small 

 pulsating vesicle is noticeable, and if the leg is 

 favourably situated, each beat of this little organ 

 may be seen to send a wave-motion down a very thin and 

 slender vessel. The colourless blood of the creature, we 

 may suppose, is thus circulating, and Mr. Buckton found 

 the pulsations from 120 to 180 times a minute. The 

 breathing is efl'ected by air-tubes (trachcf), as in other 

 insects, but their minute size makes them more difficult to 

 see than they are in larger creatures, and in such their 

 peculiar structure should be first studied. In their typical 

 form, they resemble the flexible tubes often used in con- 

 veying gas to portable lamps, which have a spiral wire 

 inside to prevent their collapsing. They glitter with a 

 silvery or pearly sheen when the light glances upon them. 

 In the aphis this wire is not discernible. The muscles of 

 these creatures are very transparent, but some of them 

 may be recognised in the legs without any colouring or 

 dissection. 



The amount of optical power possessed by these 

 creatures seems in excess of the wants indicated by their 

 habits. The young ones keep close to their parents and 

 sisters, the whole family sucking away in an increasing 

 crowd, with little room for individual motion. When an 

 entirely new settlement is to be formed, winged females 

 appear ; and with these remarks we must close for the 

 present. 



THE CHEMISTRY OF COOKERY. 



Till. 

 By W. Mattieu Williams. 



THE application of the principles already expouiided to 

 the processes of grilling and roasting is simple enough. 

 As the meat is to be stewed in its own juices, it is evident 

 that these juices must be retained as completely as pos- 

 sible, and that in order to succeed in this we have to 

 struggle with the evaporating energy of the "dry heat" 

 which effects the cookery. 



It should be clearly understood that the so-called " dry 

 heat " may be communicated by convection or by radiation, 

 or both. When water is the heating medium, there is 

 convection only, i.e., heating by actual contact with the 

 heated body. In roasting and grilling there is also some 

 convection-heating due to the hot air which actually touches 

 the meat ; but this is a very small element of efficiency, the 

 work being chiefly done, when well done, by the heat which 

 is radiated from the fire directly to the surface of the meat, 

 and which, in the case of roasting in front of a tire, passes 

 through the intervening air with very little heating ett'ect 

 thereon. 



I am not perpetrating any far-fetched pedantry in 

 pointing out this diti'erence, as will be understood at once 

 by supposing that a beef-steak should be cooked by sus- 

 pending it in a chamber filled with hot dry air. Such air 

 is actively thirsting for the vapour of water, and will take 

 into itself, from every humid substance it touches, a quan- 

 tity proportionate to its temperature. The steak receiving 

 its heat by convection, i.e., the heAt conveyed by such hot 

 air, and communicated by contact, would be desiccated, but 

 not cooked. 



This distinction is so important, that I will illustrate it 

 still further, my chief justification for such insistence being 

 that even Rumford himself evidently failed to understand 

 it, and it has been generally misunderstood or neglected. 



Let us suppose the hot air used for convection cooking 

 to be at the cooking point, as the hot water in stewing 



