138 



NATURE 



\yune 20, 1872 



and that under the river there is a natural tunnel. This 

 tunnel was described to Colonel Grant as being two months' 

 march from Unyanyembeh,and as a tunnel made by God, 

 which takes a caravan from sunrise to noon to march 

 through it. An unford able river, with rocky cliffed sides, 

 flows over the tunnel at right angles with Lake Tanganyika. 

 This river is now supposed, on apparently good grounds, 

 to be the Ruaha of Burton, and the Lufiji whose mouths 

 are shown on Captain Owen's chart. 



We now learn that Livingstone has reached Unyanyem- 

 beh, that stores are being sent up to him as rapidly as 

 possible in charge of his son, and that he will march 

 southward to explore this Ruaha or Lufiji river, this 

 mighty outlet of the great system of waters that he has 

 discovered, with its lofty cliffs and alleged natural 

 tunnel. Thus, for the third time, all fears have been dis- 

 pelled, again we get a glimpse of this true knight-errant, 

 and again we find him stedfastly working at the task he 

 set himself to do six years ago, and which he will not 

 abandon until it is finished. This last section of his 

 labours will comprise the complete discovery of the great 

 basin of the Tanganyika, including the collection of 

 accurate information respecting its limits, its varied 

 climates, its productions and capabilities and people, its 

 rivers and lakes, and its outlet to the Indian Ocean. The 

 addition to geographical knowledge will be enormous, 

 and we may well hope that this knowledge will be the 

 means by which a new country will hereafter be opened 

 to European enterprise, and the object of Livingstone's 

 life be attained. If he dies in the midst of his discoveries 

 he may well be envied, for a nobler and more glorious end 

 can hardly be imagined. If, as we all hope and as is 

 more likely, he is spared to return home, and perhaps to 

 watch in his old age the progress of the mighty work 

 which he is now initiating, he will receive a welcome from 

 his countrymen such as few have experienced and fewer 

 still have so justly earned. 



CONVERSATIONS ON NATURAL 

 PHILOSOPHY 



Conversations on Natural Philosophy. By Mrs. Marcet 

 author of '■ Conversations on Chemistry," &c. Revised 

 and Edited by Francis Marcet, F.R.S. 14th Edition. 

 (Longmans, 1872.) 



WE opened this new and revised edition of Mrs. 

 Marcet's " Conversations on Natural Philosophy ' 

 with expectation and interest ; we closed it with dis- 

 appointment and regret. The influence Mrs. Marcet 

 exerted upon the early career of Faraday, besides 

 awakening the first love for science in hundreds of the 

 last generation, will cause her name always to be remem- 

 bered with gratitude and respect. Science, however, has 

 made great strides since Mrs. Marcet wrote ; and if her 

 admirable works are touched at all, they should, where 

 necessary, be carefully and accurately revised. That 

 this has not been done in the book before us we will 

 briefly point out. Opening at the Conversation on Heat, 

 we read the following statement on p. 207 : — " It is be- 

 cause heat, light, and electricity are not subject to the 

 general properties of other bodies, and in particular to 



that of gravity, that they are commonly known by the name 

 of imponderable fluids ; " and on the next page we read 

 " that modern chemists having adopted the new word 

 caloric, to denote the principle that produces heat," we 

 are told that " caloric is found to exist in a variety of 

 forms or modifications ; and we shall consider it under 

 the two following heads, viz. : i. Free or radiant caloric. 

 2. Combined caloric. The first free or radiant caloric is 

 also called heat of temperature," &c. Again further on, 

 p. 250, the same instructor says, ''the two principal sol- 

 vent fluids are li'atcr and caloric," leading thereupon to 

 a lively conversation as to how caloric dissolves bodies. 

 This, we are told, is the way : — " Caloric, w-e may con- 

 ceive, dissolves water, and converts it into elastic vapour 

 by a process similar to that by which water dissolves 



salt It is now ascertained that the solvent 



power of the atmosphere depends solely upon the 

 caloric contained in it " ! Vivid pictures of caloric are 

 given, as "a fluid so extremely subtle that it enters 

 and pervades all bodies whatever, and forces itself 

 between their particles ;" in similar language specific 

 heat is defined, on p. 275, as " that which is employed in 

 fitting the capacity of a body for caloric, in the state in 

 which that body actually exists." Thus the minds of 

 young children for whom this hook is intended are drilled 

 into the needless and obsolete jargon of the material 

 theory of heat. 



Even the most elementary facts are often left wrongly 

 explained. Thus, on p. 255, the formation of hoar fiost 

 is accounted for in this way : — " The freezing of the watery 

 vapour, which the atmospheric heat could not dissolve, 

 produces what is called a hoar frost ; for the particles 

 descend in freezing and attach themselves to whatever 

 they meet with on the surface of the earth." We venture 

 to think there are few intelligent unscientific people who 

 could not correct this. 



We have dwelt thus far on the subject of heat, for here 

 it is that new editions of once famous books need most 

 revision. But glaring errors are to be found in other 

 parts. The diagram representing the shadow which a 

 large luminous body casts behind a small opaque body 

 (Plate 21, Fig. 3), is incorrectly shown, the converging 

 umbra only being represented without the accompanying 

 diverging penumbra. On p. 313 the luminiferous ether 

 and water are spoken of as inelastic bodies. The absence 

 of a sound shadow in air is affirmed on the same page, 

 whereas among other instances every one must have 

 noticed when watching the approach of a distant railway 

 train, how, as it winds along and is occasionally hidden 

 from view, corresponding sound shadows flit across the 

 ear. 



We have only space to indicate a kw more blunders 

 that catch our eye. In voltaic electricity the electric light 

 does not " dart from one point of charcoal to another," 

 as soon as the points are brought from " half an inch to 

 an inch " asunder. The thickness of a silk handkerchief 

 (as the writer has tried with a battery of nearly 200 cells) 

 will prevent the discharge taking place before contact is 

 made. The term " conjunctive wire," p. 428, was used by 

 Oersted, but is not now used to express the wires which 

 join the poles of a battery. In the electric telegraph 

 the current does not return through the earth to the 

 battery whence it came. This is a very popular error. 



