288 



♦ KNOWLEDGE ♦ 



[Mat 18, 1883. 



Lastly, although the words on tlio post-card are printed, 

 there are seven distinct peculiarities in the lettering, the 

 presence of all of whicli in the known writing of the 

 person who sent the card would serve to convict him of 

 his offence in any court for dealing with users of social 

 dynamite. 



I fancy that if Mr. Lockycr should join with me in 

 endeavouring to discover and expose the real offender, wo 

 might proliaMy succeed. 



PLEASANT HOURS WITH THE 

 MICROSCOPE. 



By Henry J. Slack, F.G.S., F.RM.S. 



SPRING is a good time for watching the aphides, or 

 plant-lice, in greenhouses, and also out of doors, 

 especially upon young shoots and buds of roses. A 

 favourite house-plant at this season, the so- called 

 " Marguerite," or French daisy, which is a chrysanthemum, 

 is likely to be attacked. The aphides will be seen in 

 crowds surrounding the flower-stems, which they seem to 

 like better than the leaves. Looking at some of these 

 groups with a hand-magnifier will most likely show that, 

 besides various sizes of old and young, there are some of a 

 dingier colour. These are the pupre, which produce no 

 series of budding young ones, but allow a single winged 

 female to come out, as a butterfly does from its inactive 

 chrysalis. As it is well, while writing about any such 

 matters, to have the objects under view, and to observe 

 their processes of change, several pupse were placed in little 

 cells, as described in the last paper, and watched frequently ; 

 but, as it turned out, not continuously enough for success, 

 as one of the creatures produced its imago, with four 

 delicate wings, when no one was looking, and the skin 

 from which it emerged soon shrivelled up. It is well 

 worth while to see how the imago gets out by a slit in 

 the back of the pupa. The pupa goes on feeding like the 

 larva, up to its time of change. Then a rent begins beliind 

 the eyes, which proceeds down the thorax, through which 

 the brown head of the imago emerges. The antenn.r and 

 legs are slowly pushed forward from the exuvire, or sheath, 

 which slips down the soft body of the fly by a kind of 

 peristaltic motion, " very interesting to watch under the 

 microscope." So says Mr. Buckton, and many readers of 

 Knowledge may take the trouble to witness what he 

 describes. The wings grow rapidly — so much so, that he 

 found it ditlicult to keep pace with them in working a 

 drawing. Winged females look so difl'orent from the larvw 

 and pup;i' that they would be taken for diflerent kinds of 

 insects if their origin were not known. The front wings are 

 large in proportion to the size of the little creatures, and 

 beautifully tinted with prismatic colours. The occurrence 

 of winged viviparous females allows fresh colonies to be 

 established wherever they settle. The larval aphides are 

 poor travellers, and the majority live and suck their lives 

 through close to the spot of their birth. It may be well 

 imagined that the juices of a plant ars; changed in charac- 

 ter wherever a swarm of aphides have inserted their piercing 

 and sucking tools. An entire plant soon sickens if not 

 relieved or protected against their ravages. It is supposed 

 that to facilitate a flow of the plant's juices they pour in a 

 poisonous fluid, as gnats do into our bodies, and, as the 

 character of the food is changed by the injury they do to the 

 plant, it may become loss favourable to the production of 

 young by the budding process, and more so to the produc- 

 tion of the pupal forms. Temperature has also something 



to do with the matter, but Mr. Buckton thinks the main cause 

 is the necessity of seeking food in fresh places. At first 

 only a few winged aphides are seen, but later in the season 

 large swarms are carried by the winds to considerable 

 distances. 



Rose-twigs are now (April 30) liable to such strong 

 attacks of these creatures that they may be examined with 

 a good chance of finding larvie and pupa; in various stages. 

 The latter are orange-coloured, and easily distinguished 

 from their bright green companions. In all the pupae the 

 wings of the future insect are seen in little projections, 

 like the flappers of a seal. The oggs of the rose aphis are 

 little black things, stuck into a crevice in the bark, or near 

 a bud in the winter. They hatch out in the spring, and 

 can withstand a considerable degree of cold. Mr. Buckton 

 mentions an instance in March, 1N73, when the young 

 aphides appeared, and survived, although the thermometer 

 marked 25° F. As early as the seventh hour of their age 

 he has noticed rudiments of the future young in the body 

 of the parent. 



Aphides of all kinds increase most quickly in a shelteretJ 

 situation. In greenhouses they do not like free currents 

 of air, and out-of-doors they will attack roses in verandahs 

 on the south side of a house before any seen in the operi 

 windows. Most of the nostrums advertised as killing- 

 cures of these and other plant-plagues are of no use what- 

 ever. The Gishurst Compound (a sulphur soap) is really 

 disliked by many, and it is well to wash the bark of trees 

 attacked by any of the aphis, or bug, tribe with it, or with 

 a mixture of quicklime and sulphur boiled in water. A 

 teaspoonful or two of each in fine powder to about a quart 

 of water can be cooked in an earthen pipkin. The mixture 

 should be stirred up for use upon trees, and the clear yellow- 

 fluid only employed where a stain would be inadmissible. 

 Lime and size are often used against the cotton bugs, but 

 it is the size that really kUls them. Finely-powderet) 

 sulphur, or snuff, is useful when the creatures cannot b^ 

 washed or brushed off; but I find Hardiman's Powder, sold 

 in little tin boxes with a bellows arrangement, the best I 

 have tried. The stuff looks just like Keating's well-known 

 Insect Powder, which is said to be made of a species of 

 artemisia, but it is finer. Like Keating's, its little fibres 

 stick to insects' hairs, or any roughness, and although it 

 gives off no appreciable odour to our noses, it probably 

 exhales something injurious to insects, that die, some- 

 times rather quickly, sometimes after an hour or two, but 

 are instantly injured by it 



THE BUYING POWER OF GOLD.* 



VARIOUS comments have been made by able critics 

 on the views which I have lately put forward with 



regard to the increased purchasing power of gold. May I 



be allowed to submit some further considerations to the 



public on this important subject 1 



My arguments may be briefly condensed as follows : — 

 Gold to the amount of nearly £200,000,000 has been 



required for supplying Germany, the United States, and 



Italy with new gold currencies. 



This extraordinary demand fell on a diminished supply. 



The annual production of gold during the first five years 



after the discoveries of 1S51 averaged nearly £30,000,000. 



It now amounts to less than £20,000,000. The new 



demand has been equal to the total supply of ten years. 



At the same we have to reckon with the normal demand 



* From a letter to the Times. 



