March n, 1880] 



NATURE 



447 



set them vibrating in unison with the original vibrations 

 of the radiating source. If it were not that the receiving 

 body were subjected to external influences, it would 

 acquire little by little exactly the same temperature as the 

 body from which the radiations were emitted. In other 

 words thermic equilibrium would be established between 

 the two, quite irrespective of the distance between them. 

 We know that the rays of the sun traverse space without 

 any diminution in their frequency or wave-length. It 

 follows, therefore that the sun's rays are able to raise lo 

 a temperature equal to that of the sun's surface any body 

 on the surface of the earth on which they can be concen- 

 trated, provided only such a body could be preserved 

 from losing heat by conduction or radiation. Although a 

 certain quantity of the solar radiation is arrested by 

 absorption in the imperfectly transparent atmosphere 

 surrounding the earth, measurements made at places so 

 widely apart as Cairo, Paris, and St- Petersburg agree in 

 showing almost identical values for the amount of heat 

 received from the sun, and which is about twelve calorics, 

 per square metre, per minute. 



Now on the supposition that all the metalloids, with 

 the exception, perhaps, of oxygen, are dissociated in the 

 sun, thermal equilibrium, if thus experimentally obtained, 

 ought to affect the dissociation of them upon our globe 

 also. 



M. Pictet therefore proposes that an enormous parabolic 

 mirror should be constructed, in the focus of which the 

 sun's rays should be concentrated upon the various metal- 

 loids which it is sought to decompose. All the data for 

 calculating the requisite size of the mirror are known to 

 a certain approximative value, with one exception. We 

 know the quantitative intensity of solar radiation, and the 

 reflecting power of polished metals, and hence can 

 calculate ho.v many units of heat a mirror of given size 

 will hurl into its focus per minute. We do not know how 

 much heat must be furnished to a given weight of any 

 one of the hitherto undecomposed metalloids to dissociate 

 it, but we are quite certain that this quantity must be 

 much greater than that produced by the combustion of an 

 equal weight of hydrogen and oxygen. Assuming that 

 to dissociate bromine required a hundred times as much 

 heat (at the temperature of its dissociation-point) as 

 water vapour requires (at its dissociation-point) to split it 

 up, M. Pictet calculates that a single gramme of bromine 

 must have 350 calories expended upon it to resolve it into 

 its elements. Further calculation leads him to consider 

 that to dissociate one gramme of bromine per minute, 

 would require that the solar rays should be concentrated 

 by a mirror of at least 35 sq. metres of surface, measured 

 normally to the rays, or of about ten metres' aperture. It 

 would, he thinks, be best constructed in separate pieces 

 of about a square metre in area, each ground and 

 polished to a true curve and mounted in a special frame. 

 The depth of the mirror should be equal to half its 

 aperture, bringing the focus into the plane of the rim. 

 At the focus would be a special solar chamber, or crucible, 

 constructed of lime or zircon, or other refractory substance, 

 into which the vapours to be operated upon would be led. 

 To avoid loss of heat it would be kept hot from without 

 by oxyhydrogen flames. The whole apparatus ought not, 

 he think;, to weigh as much as two tons. To catch and 

 retain the dissociated substances, and to prevent their 

 immediate recombination, he proposes to aspirate the 

 vapours of the chamber through metal tubes containing 

 metallic gauze, and cooled from without to a temperature 

 perhaps as low as— 50° by intense artificial refrigeration. 

 The rapid cooling thus produced should hinder at least a 

 considerable proportion of the constituents from recom- 

 bining as fast as they were liberated from each other in 

 the solar chamber. 



There is much that is suggestive in the proposals of M. 

 Pictet ; so much, indeed, that any attempt at criticism or 

 comment would outrun the limits of this article, which is 



therefore simply devoted to the exposition of M. Pictet's 

 ideas in phrases as nearly identical as possible with those 

 in which he has himself expressed them. 



S. P. T. 



THE DESTRUCTION OF INSECT PESTS, AN 

 UNFORESEEN APPLICATION OF THE RE- 

 SULTS OF BIOLOGICAL INVESTIGATION 

 "\A7HAT is the good of a knowledge of microscopic 

 v ' creatures ? What is the good of prying into the 

 anatomy of insects ? It is all very well as an amusement, 

 but serious persons can not be expected to assent to 

 the devotion of endowments or state-funds to such trivial 

 purposes. Chemistry, geology, electricity, if you please, 

 have their solid commercial value, but biology is an 

 amusement for children and old gentlemen." Such is the 

 opinion of many a "practical man," ignorant and short- 

 sighted as the genus invariably proves itself. 



Already the practical man may be told in reply, that 

 surgery is entirely reformed by our knowledge of the 

 minuter fungi, that by avoiding the access of Bacteria to 

 wounds, we avoid a large destruction of human life ; 

 already we see our way to avoiding some deadly diseases 

 caused by these same Bacteria now that we know them 

 to be the active cause of such disease. Already silk is 

 cheaper in consequence of our knowledge of the Bacteria 

 of the silk-worm disease ; already better beer is brewed 

 and better yeast supplied to the baker in consequence of 

 Pasteur's discovery of the bacterian diseases of the yeast- 

 plant ; already vinegar-making, cheese-making, butter- 

 making, wine-making, and other such manufacturing 

 trades are on the way to benefit by like knowledge. 

 Potato-disease and coffee-disease have been traced to 

 their causes and means suggested by biologists for 

 dealing with the parasitic plants causing those diseases, 

 whereby not thousands but millions of pounds sterling a 

 year may be saved to the community. 



Insect pests which have depopulated whole provinces, 

 such pests as the Phylloxera and the Colorado beetle, are 

 about to receive a check at the hands of the same class 

 of scientific students. The application of knowledge of 

 natural facts is in this case a very remarkable one ; for 

 it is actually proposed to make use of our recently acquired 

 knowledge of diseases due to Bacteria — not that we may 

 arrest such diseases, but that we may promote them. 

 Insect pests are to be destroyed by poisoning them 

 not with acrid mineral poisons which damage plants as 

 well as the insects, but by encouraging the spread of the 

 disease-producing Bacteria which are known to be fatal 

 to such insects. Prof. Hagen, of Cambridge, Mass., 

 has called attention to the old practice of destroying 

 greenhouse pests by the application of yeast. He 

 conceives that this method may be applied to other 

 insect-pests, such as Phylloxera, Colorado beetle, cotton 

 worm, &c. He imagines that the yeast-fungus enters the 

 body of the insect on which it is sprinkled and there 

 produces a growth which is fatal to the insect's life. It is 

 a well-known fact that insects are very subject to fungoid 

 diseases and it is also ascertained that the application 

 of yeast to the plants frequented by such insects favours 

 their acquisition of such disease. Prof. Elias Metschnikoff, 

 the celebrated embryologist, has however made some 

 investigations on this subject and given an explanation of 

 the possible value of yeast application (Zool. Anzeiger, 

 No. 47), different and more satisfactory than that which 

 Prof. Hagen appears to adopt. 



The general result of the most accurate investigations 

 of the beer-yeast fungus (Saccharomyces cerevisia), is 

 entirely opposed to the notion that it can enter an insect's 

 body and pro Juce a disease. Beer-yeast is beer-yeast and 

 appears always (or within experimental limits) to remain 

 so. On the other hand De Bary has made known the 

 life history of some simple fungi which destroy insects, 



