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HARD WICKE ' S S CIE NCE - G SSI P. 



or absence, or is it inherent ? If the latter, we may 

 partly account for the rapid disappearance of the 

 great broods of little frogs by parental voracity. I 

 am told that the great prosperity of the Medical 

 School of the University of Edinburgh has created a 

 serious scarcity of frogs in the suburban districts of 

 the Scottish metropolis. 



The Canals of Mars.— A few years ago (1S81 

 and 1882) astronomers were puzzled by the announce- 

 ment of a strange discovery by Schiaparelli, who 

 stated that the planet Mars displayed, besides the 

 polar snow-caps, the islands, peninsulas, seas, fjords, 

 and continents previously observed, a number of 

 canals or lines extending over great distances on 

 the surface of the planet, mostly in parallel pairs, like 

 exaggerated railways. As they had not been seen by 

 other observers, and were still invisible to the 

 majority, considerable doubts arose concerning their 

 existence, especially as no physical explanation of 

 their origin could be assigned without indulging the 

 imagination very freely. At a recent meeting of the 

 Academy of Sciences M. Faye announced that M. 

 Perrotin and other observers at the Nice Observatory 

 have redetected the canals of Schiaparelli. They 

 appear to vary in shape and visibility with the seasons 

 of the planet. 



In describing the question of the reality of obscure 

 telescopic details on the surfaces of planets, it should 

 be always noted that the visibility of such faint 

 markings depends far less upon the magnifying power 

 of the telescope used than upon the definition ; and 

 as these markings are all more or less foggy and 

 ghost-like, they are easily obliterated by very slight 

 atmospheric obscuration. A mist which is itself in- 

 visible and insufficient to sensibly obscure a star may 

 quite blot them out from sight. 



It is a significant fact that Italian astronomers such 

 as the two Cassinis, De Vico, Bianchini, etc., should 

 long ago have described details on Venus and other 

 planets, which Schroter confirmed, but which our 

 English astronomers, with far better telescopes at a 

 later date, could not detect. I have no doubt that 

 the advantage of Italians and Germans was due to 

 the superior clearness of the atmosphere in which 

 themselves and their instruments were immersed. 



I have discussed this question in chapter fifteen of 

 " The Fuel of the Sun," which treats of the meteor- 

 ology of the moon and the inferior planets, and here 

 summarise an experience there described in detail. 

 During a sojourn at Valetta, where, in accordance 

 with Maltese custom, I spent many evenings on the 

 house-roof, I found that the white summit of Mount 

 Etna, a hundred miles distant, was just visible in very 

 clear weather. It was only with great care, aided 

 by familiarity with its shape, that this became possible 

 with the naked eye. On using a small hand telescope 

 it was lost. The difficulty of distinguishing it was 

 not due to want of dimensions, nor want of light, but 



to the faint nebulous character of the outline, which 

 differed so slightly from the surrounding haze of the 

 horizon, which even there was sufficient to hide the 

 cone in ordinary bright, cloudless, Mediterranean 

 weather. Exceptional clearness, even of that sky, 

 was required. It is doubtful whether, in our climate, 

 such an object at such a distance could ever be seen. 

 The features of the telescopic physiognomy of Mars, 

 Venus and Mercury are similar in character to the 

 outline of a snow-peak all but veiled in the sea-haze 

 of an Italian summer horizon. The slight additional 

 obscuration due to the imperfect transparency of the 

 telescope lenses was the last optical straw that broke 

 down the possibilities of vision in the case of Mount 

 Etna. 



The Three Conditions of Matter. — Oxygen 

 has now been distinctly solidified. Professor Dewar 

 has completely succeeded in obtaining it in the form 

 of snow at a temperature of 200 below zero centi- 

 grade, or 360 below zero Fahrenheit. We may now 

 safely assert the generalisation which was formerly but 

 speculative, viz. that all elementary matter may exist 

 in either of the three conditions, of solid, liquid, or 

 gas, and that these conditions simply depend upon 

 temperature and pressure. I say " elementary 

 matter," because the case of compounds is different. 

 Many, such as water, are familiarly known in all the 

 three states, but others become dissociated ; their 

 elements are driven apart by heat at a lower tem- 

 perature than that which is necessary for vaporising 

 them. Sugar, for example, is readily fused into a 

 liquid by heat, but if we attempt to volatilise it we 

 find that it darkens in colour, and finally becomes a 

 black carbonaceous cinder. Sugar is one of the carbo- 

 hydrates, or compounds of carbon and water ; the 

 heat drives off the water as steam, and leaves the 

 carbon behind. 



The fixed oils are similarly dissociated, but in this 

 case only just before they arrive at their boiling-point. 

 Some of these may, however, be distilled without 

 decomposition, by heating them under diminished 

 atmospheric pressure. 



Stroh's New Stereoscope. — Many years ago, 

 when the stereoscope was a novelty, I tried some vain 

 experiments for the purpose of obtaining a stereoscopic 

 picture on a screen by various methods of super- 

 posing right and left eye stereoscopic photographs 

 Others have probably attempted the same, and have 

 similarly failed. On the 1st of April (a date that 

 would have been quite appropriate for my experi- 

 ments) a new form of stereoscope was described to the 

 Royal Society by A. Stroh. His apparatus, like mine, 

 consisted primarily of two dissolving-view lanterns 

 placed side by side, by means of which he superimposed 

 the two pictures on a screen. In front of his lantern 

 lenses is a rotating disc, portions of which are cut 

 away, so that during its rotation it obscures the light 



