Aprils, 1880] 



NATURE 



54i 



long-felt want of a good and cheap blue colouring matter, 

 was at first kept secret, till the process of preparation was 

 published in a paper in the Philosophical Transactions, 

 1724, by Woodward. Afterwards a number of chemists 

 attempted to ascertain the true nature of the colouring 

 matter, but without any further success than that improve- 

 ments were made in the process of preparation, and some 

 new substances were discovered, into the composition of 

 which the colouring principle entered as a constituent — 

 among which as the most important may be named the 

 yellow prussiate of potash discovered by Macquer in 1752. 

 In 1772 Sage still believed that phosphoric acid entered as 

 a constituent into the composition of prussiate of potash, 

 and in 1786 Westrumb declared the colouring matter to 

 consist of a volatile phosphor-soap. Two short papers by 

 Scheele, together occupying not morethat twenty-two pages 

 of the Transactions of the Swedish Academy of Sciences 

 for 1782 and 1783, at last brought clearness into this con- 

 fusion. Scheele showed that the origin of the colouring 

 power was a peculiar volatile acid (blue acid), whose 

 properties he accurately ascertained, and of which he 

 produced many new and important compounds. Scheele, 

 besides, showed that the acid contained carbon and 

 nitrogen, and that when subjected to combustion it gives 

 off carbonic acid, and to dry distillation ammoniacal salts 

 among others. On the other hand, Scheele is thought 

 not to have suspected that the substance which, without 

 any special precautions, he produced in considerable 

 quantity, smelled and tasted, forms one of the most 

 powerful poisons with which we are acquainted. The 

 work thus begun by Scheele was carried on by later 

 chemists (Porret, Gay-Lussac, Berzelius, Faraday, Gmelin, 

 Wohler, Liebig, &c), and it is perhaps not too much to 

 say that the accurate knowledge we have thus obtained 

 of the nature of the compounds of cyanogen has exerted 

 an influence on the development of chemistry only in- 

 ferior in importance to that of the discovery of oxygen 

 and chlorine. 



We find the first printed work of Scheele in the Trans- 

 actions of the Swedish Academy of Sciences for 1771. It 

 bears the title, " On Fluor Spar and its Acid." Among 

 the treasures which Pompey brought with him to Rome 

 after his victory over Mithridates are enumerated goblets 

 and cups of a beautifully-coloured fragile mineral, which, 

 from the town at the plundering of which they were first 

 obtained, were named Vasa myrrhina. They soon be- 

 came fashionable, and Pliny and other writers speak of 

 the fabulous sums that were paid and the bloody deeds 

 that were done in order to obtain them. Among all the 

 different objects belonging to Ancient Rome preserved in 

 our museums, there is not, remarkably enough, a single frag- 

 ment of these vessels so renowned in the history of luxury ; 

 but it has been guessed that they were made of a mineral 

 which is now called fluor spar, and is still occasionally 

 used for vases and cups. In mineralogical literature pro- 

 per this mineral is first mentioned by the Saxon Agricola, 

 who speaks of its use as a flux in the smelting of metals, 

 and warns against mistaking this beautifully-coloured, but 

 brittle and by no means hard mineral for a precious stone. 

 Somewhat more than a century afterwards the art of 

 etching on glass by means of this mineral was discovered 

 at Niirnberg, and about the same time Elsholz, a Berlin 

 physician who employed himself in the examination of 

 phosphorescent substances, discovered that fluor spar 

 became self-luminous when heated. All substances that 

 are self-luminous without sensible combustion attracted 

 at that period immense attention — certainly a survival 

 from the alchemists' dreams about the philosopher's stone, 

 which, among its other perfections, was also to have that 

 of being self-luminous. The mineral, in consequence of 

 its property of being a "light-bearer," now became the 

 object of repeated but rather resultless examinations, until 

 Scheele, by some simple experiments both analytic and 

 synthetic, showed that the mysterious substance consisted 



of "lime saturated with a peculiar acid." This acid 

 attacks glass and dissolves with ease siliceous substances. 

 Scheele at first employed glass-vessels in producing it, 

 which gave occasion to erroneous statements. The mis- 

 takes were immediately acknowledged and corrected 

 when Scheele's attention was drawn to them by the 

 apothecary Meyer of Stettin. Unwarranted objections, 

 for instance by the Frenchman Boulanger, who declared 

 that fluoric acid was nothing else than muriatic acid 

 combined with some earth, and by Monnet, who believed 

 that fluoric acid was only vitriolic acid " volatilised by 

 means of a singular combination with fluor spar," were 

 on the other hand refuted by new experiments in a paper 

 printed in the Transactions of the Swedish Academy 

 of Sciences for 1780. Scheele's examination of fluor 

 spar had as its direct result the discovery of the simple 

 substance fluorine, which without doubt, through its 

 general occurrence in nature, and its properties differing 

 from those of all other simple substances, is destined 

 to play a very important part in the development of 

 chemistry, although our knowledge of it is yet very in- 

 complete from the impossibility of procuring vessels 

 capable of resisting its corroding action. 

 {To be continued) 



ON THE LONG PERIOD INEQUALITY IN 



RAINFALL « 



1. T F it be true that there is a variation in the power of 



■^ the sun depending on the state of his surface, this 

 variation might naturally be expected to make itself 

 apparent through a corresponding change in the rainfall 

 of the earth, so that when the sun is most powerful there 

 ought to be the greatest rainfall. 



2. While the connection indicated above is that which 

 most readily occurs to the mind, yet the difficulty of 

 ascertaining the facts of the case in a manner bearing the 

 smallest approach to completeness, is so great as to be at 

 present insuperable. 



There is, first of all, an intense reference to locality in 

 rainfall, so that the rainfall at one place maydiftti giwii; 

 from that at another place in lis near neighbourhood. 



Again, there are, probably, in addition to possible 

 secular inequalities, very great oscillations in the yearly 

 rainfall at any one place, or accidental variations as we 

 may term them, in our ignorance of their cause. 



Thirdly, it is in comparatively few places, and those 

 places chosen not with the smallest reference to this 

 particular problem, that we have anything like a trust- 

 worthy account of the rainfall throughout a considerable 

 number of years. 



Fourthly, we have no information of any importance 

 with respect to the rainfall at sea. 



3. Besides the formidable catalogue of difficulties now 

 mentioned, we ought to bear in mind the following con- 

 siderations. The convection currents of the earth are 

 regulated by two things, one of which is constant, while 

 the other may be variable. The constant element is the 

 velocity of rotation of the earth on its axis, while the 

 element of possible variability is the power of the sun. 

 Hence it follows that if the sun be variable it will cause a 

 variation in the direction as well as in the intensity of the 

 earth's convection-currents on the principle which tells 

 us that the resultant of two forces, one constant and the 

 other variable, must vary both in magnitude and direction. 



Now if it be true that we have a long period variation, 

 not merely of the intensity, but also of the distribution of 

 the earth's convection-currents, and if we bear in mind 

 the intensely local reference in rainfall, it would be too 

 much to expect that the rainfall inequality should exhibit 

 the same years of maximum and minimum at all places. 



' By Balfour Stewart, I.L.D., F.R.S., Professor of Natural Philosophy at 

 the Owens College. Manchester. Being a paper read before the Lit. and 

 Phil. Society of Manchester. 



