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NATURE 



[Nov. 4, 1875 



SCIENCE IN GERMANY 



(From a German Correspondent^ 



TTERR NEESEN, assistant to Prof. Hclmholtz, has 

 ■*• ■*■ recently published a memoir on the phenomena of 

 attraction and repulsion by light and heat-rays, observed 

 by Mr. Crookes. He states in it that he had already for 

 two years observed such phenomena, which at fust 

 seemed to be a case of mechanical action of light-rays, 

 i.e. of the effect of their impinging and rebounding on the 

 surface of a mirror suspended by a cocoon-fibre. He 

 now thinks, however, from the experiments he has made, 

 that the effects of light and heat-rays in question are to 

 be regarded as merely produced through air-currents 

 arising from heating of the air in certain parts of the 

 apparatus in which the movements of the mirror take 

 place. Neesen first shows that in the phenomena ob- 

 served such air-currents have, in fact, influence. He 

 used for his experiments a rectangular case of sheet iron, 

 in the upper cover of which was a peculiar arrangement 

 for hanging a cocoon fibre. In the lower part of one of 

 the sides of the case was a rectangular aperture closed by 

 a plane parallel glass plate, and behind this plate was the 

 suspended mirror. The air-currents above referred to 

 arise not only from the fact that the air in contact with 

 the glass plate through which the light must pass to reach 

 the mirror, or the air in contact with the mirror, is heated. 

 The air-particles also between glass plate and mirror are 

 heated by conduction of the heat ; and so, by their heat- 

 ing also, air-currents are produced which tend to turn the 

 mirror. 



In favour of Neesen's explanation are the facts (i) that 

 the movements of the mirror always decrease when the 

 air in which it is enclosed is rarefied ; and (2) that these 

 movements also become less if adiathermanous sub- 

 stances {e.g. a column of water) absorb the light-rays 

 before these can reach the mirror. 



The considerations which seem to be against , his ex- 

 planation are the following : — (i) The reversal of the 

 movement, observed by Mr. Crookes, on a certain small air 

 pressure being reached ; (2) a fact appearing from Neesen's 

 own experiments, viz., that according as the direction in 

 which the light-rays fell on the mirror was varied, was the 

 direction of rotation of the mirror changed, though the 

 light fell on the same part of the mirror. Both these 

 peculiarities, however, may also be explained by air- 

 currents. First, as regards the fact observed by Neesen, 

 it is clear that, according as the lamp is placed to one side 

 or the other of the mirror, different parts of the glass 

 plate in front are heated, and different parts of the air- 

 layer between glass plate and mirror ; and accordingly 

 the currents and the rotation of the mirror must have an 

 opposite direction. To explain the reversal of the motion 

 on a certain low pressure being reached, Neesen calls to 

 mind that the conductivity of air for heat, as Kundt and 

 Warburg have shown, decreases with extraordinary ra- 

 pidity on decrease of pressure (well observed with low 

 pressures) ; so that with as perfect a vacuum as possible, 

 it entirely disappears, and only radiation of heat remains. 

 Now, as long as the heat is conducted, the air-particles 

 conducting the heat are themselves heated. On the 

 other hand, no such heating takes place when the heat 

 passes only by radiation. That with such very different 

 conditions the currents may be different is probable in a 

 high degree. W. 



MULLER ON BEES AND FLOWERS* 



J N this communication Dr. Miiller calls attention to the 

 ■*• interesting facts presented by various groups of 

 Hymenoptera, in which we find a series of forms pre- 

 senting more and more complex life relations, accom- 



* " Die Bedeutung dM Honigbiene fur unsere Blumen," in the Bieneit 

 Zeitung for July 15. 



panied by a higher and higher mental organisation. 

 The consideration of these gradations is calculated to 

 throw much light on the question, " How has the Money 

 Bee acquired its remarkable instincts ?" a question which 

 the study of that species alone would, in his opinion, do 

 little to solve, but on which the habits and organisation 

 of other groups throw much light. The Sawflies ( Ten- 

 thrcdo) are amongst the lowest of Hymenoptera. They 

 merely choose a plant of the species on which they have 

 themselves lived, cut a hole in the leaf with their curious 

 saw, and deposit therein an egg. The young larva; thus 

 find themselves on their tood and live like ordinary 

 caterpillars, which in general appearance they much 

 resemble, and like which they are exposed to destruction 

 by various enemies. 



Passing on to the Gall Insects (Cymps), we meet with 

 a new mode of life which is very instructive. The in- 

 cision made in the plant by the Sawfly causes little 

 abnormal growth, while in the case of the Cynipida;, on 

 the contrary, it gives rise to the well-known galls. 



Some species, however, pierce not plants, but animals, 

 and have thus opened out for themselves many more 

 possibilities of existence, since there is scarcely any 

 group of insects which is free from these attacks ; neither 

 the thick-skinned beetle, nor the active and powerful 

 wasp, nor the woodboring larvae of Cerambyx, nor even 

 the aquatic larvje of the Phryganeas. 



This passage from phytophagous to carnivorous habits 

 has not only led to the formation of many new species, 

 but also to a greater complexity in the relations of the 

 parents to their young, and to a higher intellectual deve- 

 lopment, which is shown especially in the arrangements 

 made for the nourishment of the larvae, since it certainly 

 requires both greater energy and more intelligence to 

 discover and attack a particular species of insect than 

 merely to lay an egg on the plant which has served the 

 mother herself for nourishment. The passage from the 

 gall insects to these insect-piercing species must, in M. 

 Miiller's opinion, have been slow and gradual. The 

 genus Synergus, which deposits its eggs in the galls ot 

 the true gall insects, constitutes, perhaps, a link between 

 the two groups. 



On the basis of this increased energy, intelligence, and . 

 adaptability, certain groups then made a still further 

 advance by which some of the drawbacks incident to 

 such a mode of life were avoided. For it of course fre- 

 quently occurs that caterpillars and other insects in 

 which these insect-piercing Hymenoptera have deposited 

 their eggs, are devoured by birds or other enemies. Cer- 

 tain species, however, meet this danger by transporting 

 their victims to a place of security. To effect this, how- 

 ever, certain conditions are necessary. The aggressor 

 must be sufficiently large to overpower his victim, but the 

 latter must not be killed, or it would decay and thus be- 

 come unsuitable for food. Dr. Miiller considers that 

 many insect-boring species have probably endeavoured 

 to secure their prey, but have under these circumstances 

 found it impossible to do so. Thus, the ovipositor of the 

 Tenthredo became the sting of the wasp, and thus those 

 species which carried off their victim to a place of con- 

 cealment would abandon the habit of laying their eggs 

 inside the victim. Dr. Miiller expresses the opinion 

 that the various proceedings by which the solitary wasps 

 thus protect their young against contingencies to which 

 the insect-piercing species are liable, must have at first 

 been arrived at with a consciousness of the object to 

 be effected, but that they have gradually become in- 

 stinctive, and are now unconsciously inherited from gene- 

 ration to generation. Still it is, he observes, impossible 

 to watch a wasp at work without feeling that, with these 

 inherited customs, or so-called instinct, much individual 

 effort also comes into play. Dr. Miiller proposes to discuss 

 this interesting part of the subject in detail in a future 

 communication. J. L. 



