Dec. 2, 1881.] 



KNOWLEDGE 



85 



i ^ MAG^dNEOF^tyENCE^ I 



I PLAfNrrVfORDED-EXACTDflDESCRlBED | 



LOyDOX: FRIDAY,-! DECEMBER 2, 1881. 



CONTENTS. 



PAGE. 



tmi- Kii-liis. Bv E. W. Prevost 85 



Comen anil Cornels' TaUs. By the 



Kdilor— (/«"•'••'■'"') ■"• 88 



Solids, Liquids, and Gases. — Part 



III. Bv W. Mattieu Williams ...8/ 

 Germs of' Disea.«e and Death. Bv 

 Dr. Andrew WUson, F.R.S.E.— 



Part II «1 



Brain Troubles. — Impaired Memory IH) 



Man a Fruit-eater 91 



.1 .Mild November 92 



CoiiREapo>l>ESCE. — The Invisibility 

 of Light — Inclination of the 

 Earth's Axis— The Zoetropc— The 



PiGE. 



Missing Link— The 1-indi Ord- 

 nance Slaps -Palizsch & Halle.v's 

 Comet — Do Comets obevGraritv? 

 —The Crimson - circled Star- 

 Tables of Meridianal Parts, &c .... 93 

 The Eastern Skies in December — 



(nimtraled) 97 



Queries 101 



Replies to Queries UK 



Training 101! 



Our Mathematical Colunm 103 



Our Chess Column IM 



Oar ■must Column 101 



Answers to Correspondents 106 



OUR FIELDS. 



By E. \V. Prevost. 



IT is not probable that very many of the readers of 

 Knowledge have ever thought, when walking tlirough 

 tlie fields, how those fields came to be in the condition in 

 which tliey now are ; not as regards the crops which they 

 bear, but as regards the fine powdery state of the earth, 

 when below are only to be found hard rocks. We do not 

 propose to enter into the question why the rocks below 

 occupy their present position, but rather to consider some 

 of the means whereby rocks generally have yielded, and still 

 do yield, a material difierent in character to themselves, and 

 one suited to the gi'owth of plants. For simplicity's sake 

 we will assign the origin of a soil to two rocks known as 

 Granite or Trap ; for although a soil is not wholly nor 

 directly derived from these two, yet tiie changes, such as 

 those experienced by granite and trap, are the same in 

 character, though differing in degi'ee, in whatever be the 

 rock under consideration. 



Granite, a substance produced by the agency of heat in 

 the early geological ages, is a mixture of three different 

 minerals — quartz, a white lustrous mineral, also known in 

 certain forms as rock-crystal, fel.'^par, and mica. Felspar, 

 of which there arc several kinds, is a silicate of the metal 

 aluminium, together with the silicates of potash, lime, or 

 soda, according to the variety of felspar. Slica appears in 

 the form of small bright shining scales, and is a silicate of 

 potash or magnesia. Together with these three minerals, 

 which may be called the elements of tlie soil, there are 

 always present small quantities of other substances, sucli 

 as iron. Sec. 



The trap rocks, divisible into two great classes, called 

 dioiite and dolerite, contain soda, lime, magnesia, and 

 potash ; and phosphoric acid is also present, but not to a 

 large extent. Here, then, we have the materials necessary 

 for the formation of soil, but its quality is dependent not 

 only on the composition of the minerals contained in the 

 rocks, but also upon the proportion in which they have 

 contributed their constituent parts. The first agency to 

 which the disintegi'ation of a rock is due, reducing it from 

 the original massive condition to that of a powder, is frost 



aware of the fact that when water freezes, it expands to a 

 considerable extent at the moment of solidifying, and 

 that this expansion is irresistible, the vessels en- 

 closing the water being destroyed. This same iiTe- 

 sistible force is continually at work during the winter, 

 destroying the cliffs on the sea-shore and iiJand ; for 

 there are always a number of clefts or fissures that 

 ridmit of the entrance of water, which, when the tempera- 

 ture is low enough, is there frozen, and in freezing expands 

 and splits the stone : and then, if the position be favoui'able, 

 this splmter from the main rock falls down as soon as the 

 ice melts, in consequence of a rise of the temperature. 

 Again, the alternations of heat and cold, without the inter- 

 vention of water, produce rifts, because the rock, being 

 composed of substances which are not affected to the same 

 extent by the same rise of temperature, expands unequally, 

 and the" result is the separation of the parts from one 

 another. It is thu.s that the bed.s of torrents, rushing down 

 glens, are in part tilled by stones of various sizes, which, 

 raising the level of the bed at the same time by theii- re- 

 moval from the banks, alter the character of the glen. 

 Even before the boulders become detached from the cliff, 

 they are subjected to a slow but sure process of 

 destruction, for every drop of rain which beats against the 

 bare stone has its effect in wearing the surface away. In 

 these processes of destruction w-e recognise mechanical 

 agencies : but this is not all ; chemistry plays also a very 

 important part in reducing a block of stone to powder. 

 The rain, in falling through the air, dissolves some of the 

 oxygen and carbonic acid gas which it contains, and then, 

 when this rain, holding carbonic acid in solution, comes in 

 contact with the rock, a part of the more soluble portions 

 is dissolved. It is, therefore, evident that the rain is a very 

 powerful destroyer of stone, in that it works in two distinct 

 ■ways — mechanically, whereby the rock is powdered, and 

 chemically, whereby a part is carried away in solution. 



Now, let us see what is the result of this partial solution 

 of an apparently insoluble material. In the case of granite, 

 Ijearing in mind that there are at least three different sub- 

 stances present, which are firmly and solidly mixed together, 

 simple powdering would result in a separation fi-om one 

 another of the quartz, felspar, and mica ; and if a cm-rent 

 of water be passing over the rock at the time, the heavier 

 particles would be left behind and the lighter ones would 

 be swept away. This is what actually occurs : the quartz, 

 finally known as sand, is left behind, while the lighter 

 particles of felspar and mica are removed ; now, seeing that 

 tlie water, whether rain or river, contains carbonic acid, 

 the felspar is slowly acted upon by the acid, is decomposed 

 into silicate of alumina, and potash, which passes into solu- 

 tion, while the silicate of alumina remauis behind or sus- 

 pended in the water, and is now known as clay. As for the 

 mica, it is not easily afl'ected chemically, and may either be 

 left behind wnth the clay, or, if the stream be rapid, it is 

 carried away and deposited elsewhere. Such a process as 

 is here described explains the destruction of a rock, and the 

 consequent formation of beds of sand and clay, which are 

 ill-fitted for the growth of plants, and to explain tliis we 

 pictured to ourselves the rock in its original position, and 

 undergoing all the pi-ocesses at one time. To account for 

 the production of loam, it is not necessarj- that every pro- 

 cess should occur at once ; but rather that, firstly, the 

 boulder, removed by frost from the main rock, should be 

 further broken up in the bed of a stream, and the particles 

 so produced be then can-ied down by the stream and de- 

 posited where the current is less swift ; this deposit, in 

 time, becomes bare of water, as the stream alters its course, 

 and finally becomes dry land. Now our new soil is not 

 vet fit for nlants. and as it consists of particles of the 



