July 1, 1891.] 



KNOWLEDGE 



127 



about by pressure, when the volume of the bodies formed by 

 decomposition is less than that of the compound. Briefly, 

 pressure brings about such chemical changes as are accom- 

 panied by a contraction. The apparatus employed in these 

 experiments consisted of a small steel chamber, in which the 

 substances were placed, furnished with a piston worked by a 

 powerful lever, provided at the end with a heavy weight. If 

 the piston were forced down rapidly the substances would be 

 heated, and it would be impossible to discriminate between 

 the changes due to rise in temperature and those due to 

 increased pressure. In Spring's apparatus the lever is 

 lowered very gradually, its descent being regulated by a 

 finely cut screw. The steel chamber is surrounded by 

 water, in which is placed a dehcate thermometer, and the 

 descent of the piston is operated so gradually that there is 

 practically no rise of temperature. These experiments 

 aflbrded the first example of the direct conversion of 

 mechanical work into chemical energy. 



As a last example of the application of experimental 

 methods in Geology, we will deal with some of the problems 

 presented by mineral veins. The cracks and openings by 

 which rocks are traversed are in some cases unfilled, in 

 others they contain debris of the rock itself, and lastly, 

 they are sometimes found filled with foreign minerals, and 

 are then known as mineral Veins. Most of the fine well- 

 crystallised minerals which adorn museum collections come 

 from mineral veins, or from ca\dties in rocks, known as 

 " geodes." Tin veins, for instance, contain the oxide of tin, 

 cassiterite, in large well-formed crystals. Others contain 

 oxide of iron, also well crystallised ; and another class con- 

 tain the metallic sulphides, such as galena, the common lead 

 ore found in Derbyshire and elsewhere. The processes by 

 which these minerals have accumulated in the veins, and 

 the mode m which their crystallisation was induced, long 

 remained a mystery. In studying the characters of tin 

 veins, Daubree was struck by the constant presence of 

 minerals, such as apatite, topaz and tourmaline, which 

 contain the elements chlorine and fluorine. It is known 

 that the chloride and fluoride of tin are volatile, and that 

 these compounds are decomposed by water, the hydrogen 

 of the water forming hydrochloric or hydrofluoric acid gas 

 and the oxygen combining with the metal. Experiment 

 showed that if the vapour of water be brought in contact 

 with that of chloi-ide of tin at a fairly high temperature 

 the oxide of the metal is formed in crystals, having all the 

 chara<cters of the mineral cassiterite. Later, Sainte- 

 Clau-e Deville showed that by passing hydrochloric acid 

 gas over strongly heated oxide of tin, the chloride of the 

 metal was formed, and steam. These two gases are carried on 

 by the current of hydrochloric acid gas, and in a somewhat 

 cooler part of the apparatus react again upon one another, 

 re-forming hydrochloric acid, and depositing the oxide of tin 

 in the crystalline form. Hydrofluoric acid and other re-agents 

 which have been found to act in a similar manner have 

 been termed mini'riilii«iteiirs. They are capable of eflecting 

 the transport of non-volatile substances, such as oxide of 

 tin, depositing them m the crystalline form, and, their 

 work done, they leave no trace of their former presence 

 except in the combination of their more active elements 

 with constituents of the surrounding rock or of certain 

 minerals of the vein, the gauque minerals as they are 

 termed. One of the commonest of the gauque minerals is 

 calcite, and by passing the vapour of chloride of phos- 

 phorus over heated calcite, Daubree found that apatite is 

 formed, a mineral which, as has been said, is characteristic 

 of tin veins. In the formation of the metallic sulphides, 

 sulphuretted hydrogen has played the part of miwralisntiitr. 

 In some cases, no doubt, the mineralising action is effected 

 by the vapour of the active substance ascending from the 



heated interior ; in other cases, as in the neighbourhood 

 of the great Compstock Lode, the substances are in 

 solution, subterranean streams saturated with sulphuretted 

 hydrogen and other powerful solvents serving to extract 

 the metallic compounds from the rocks in which they 

 are present in small quantities and concentrating them 

 in the vems. By such processes metallic ores are collected 

 in the rock veins, which thus become the great store- 

 houses of mineral wealth. 



FLYING ANIMALS. 



By E. Lydekker, B.A.Cantab. 

 [Continued from iirii/e 115.) 



THE wing of a Bird, although constructed on the 

 same fundamental plan as that of a Pterodactyle, 

 difiers altogether in the maimer in which the bones 

 corresponding to those of the human hand have 

 been modified for the purposes of flight. In the 

 wing of a Bird only three fingers are represented at all, 

 and these probably correspond to the thumb, index, and 

 middle fingers of the human hand. Moreover, while 

 the thumb is only represented by a small splint of bone, 

 which carries the so-called "bastard wmg," the bones 

 of the index finger are flattened, and much larger than 

 those of the third one, with which, in living birds, 

 they are more or less closely united. This finger, 

 therefore, forms the chief part of the extremity of 

 the wing or pinion ; but, instead of its bones being much 

 longer than those of the arm and fore-arm put together, 

 as is the case with the elongated outer finger (-ith or oth, 

 as the case may be) of the Pterodactyle, it is much shorter, 

 and, indeed, is frequently shorter than either the bone of 

 the upper arm or those of the fore-arm alone. The finger, 

 therefore, is the least important part of a Bird's wing, 

 whereas the outer finger is by far the most important 

 element in that of a Pterodactyle. 



It would involve a large amomit of detail to give a fuU 

 description of the arrangement of the feathers of a bird's 

 wing; and it must accordingly suttice to say that the 

 large flight-feathers carried by the pinion are known as the 

 primaries, while those attached to the larger bone of 

 the fore-arm are termed srcondaries ; the smaller feathers 

 which overlie these being designated as the iriii;i-ciirfrts. 

 In all living birds, as we have said, the bones representing 

 the fingers are flattened, and those of the mdex and third 

 fingers more or less united together. In the Anlwoptenjx, 

 which is the oldest known bird and a contemporary of the 

 Pterodaotyles of the Lithographic Limestones of Bohemia, 

 all the three fingers were, however, perfectly separate from 

 one another, and each ended in a claw ; while the index 

 was not greatly larger than the other' two. We have, 

 therefore, in this bird a decided approach to reptQes ; 

 fi-om which class it is considered that birds were originally 

 derived. 



In addition to their wings we must not omit to mention 

 that the tails of birds form an important aid in flight, 

 when they act as a kind of rudder. In all living birds the 

 bones of the tail are extremely few in number, and the 

 large tail-feathers all take origin close together, and are 

 generally spread out in a more or less fan-like manner. 

 Our old 'friend the Arch;i'opteryx had. however, a very 

 long lizard-like tail, with a pair of feathers arising irom 

 each of its numerous jomts, after the mamier of the 

 feathers on an arrow. It will, however, be readily 

 imagined that such a long unwieldy tail was by no means 

 calculated to act as an ellicient and compact rudder ; lUld 

 the shortened tails of modern birds appear, therefore, tiO 



