April 12, 1888] 



NATURE 



555 



crystals break rather than become detached. It also 

 varies with physical conditions, as, for instance, the 

 more or less polished condition of surfaces in contact. 

 The total adhesion is evidently proportional to the 

 extent of surfaces in contact : it will be so much the 

 greater as (i) the volume of empty spaces due to excess 

 of water employed in mixing is less ; (2) as each crystal 

 for a given weight of matter presents a great extent of 

 surface (the form of elongated prisms recognized in the 

 crystallization of plaster and of all similar products is 

 eminently favourable to the development of adhesion) ; 

 (3) as the crystals are grouped so as to increase the 

 volume of empty spaces and so as to diminish their number 

 and isolate them one from the other. A structure like 

 that of pumice is particularly favourable to strength. The 

 nature of the solvent, temperature, and nuclei of crystal- 

 lization, all serve to modify considerably the growth of 

 crystals, and consequently to influence in a like degree 

 the strength of the mortar. A study of barium silicate 

 has led to the conclusion that its hydration may take 

 place in a manner denoted by the following equations : — 



BaSiOg + 6H2O = BaSiOs • 6H2O 

 Ba2Si04 + 1 5H.,0 = Ba2Si04 • 6H2O + BaO • gH^O 

 SiOs + BaO • 9H2O = BaSiOa • 6H2O + 3H2O. 



The setting of siliceous baryta cements is due to the 

 production of the same hydrated silicate, BaSiOg " 6H2O, in 

 whatever manner it may be formed. 



Mortars and Hydraulic Cements. — Calcareous mortars 

 are divided into two classes : air-dried mortars ; hydraulic 

 mortars and cements. 



Air-dried mortar is made from quick-lime slaked with 

 water and mixed with sand. As Vicat has shown, the 

 first stage of its setting is caused by the desiccation of 

 extremely fine particles of lime, and is identical with the 

 hardening of clay. The sand acts as jn the making 

 of bricks — it prevents too much shrinkage by forming 

 an incompressible base or body. A further degree of 

 hardening is caused by the conversion of the lime into 

 carbonate. 



The burning of limestone, unlike the dehydration of 

 gypsum, is the result of the phenomenon of dissociation, 

 as was proved by Debray. Various kinds of lime all 

 become burnt at 850° C. 



Hydraulic Cements. — These are obtained by baking 

 natural or artificial mixtures of lime and clay, containing 

 from 21 to 27 per cent, of clay. 



They are divided into slow-setting and quick-setting 

 cements. The former are baked at a much higher 

 temperature than the latter. The setting of the former 

 proceeds for some hours, and much facilitates their use. 



There appear to be three different anhydrous cal- 

 cium silicates, of which one only, the tricalcic silicate, 

 SiOg • 3CaO, is attacked by water, and is capable of 

 setting ; there are three calcium aluminates, which all 

 set very quickly after mixing with water ; there are 

 ferrites of lime, which slake and swell out like quick-lime, 

 and numerous other compound silicates which are more 

 or less unalterable by water. 



The only hydrated salts which can exist in presence of 

 an excess of lime, and which are formed from the above, 



are: — 



Silicate, SiO^ • CaO • 5H2O. 

 Aluminate, h\0^ ' 4CaO * I2H,0. 

 Ferrite, FejOg ' 4CaO • Aq. 



A microchemical study of anhydrous cements has 

 shown that there are colourless crystals of a pseudo- 

 cubic or hexagonal appearance. The intervals between 

 these crystals are filled with coloured matter without 

 crystalline character, which has been in a state of fusion. 

 The crystals are formed by chemical precipitation in the 

 midst of the brown fusible matter which afterwards solidi- 

 fied on cooling. The composition of the crystals is that 

 of a calcium silicate, and of the amorphous portion of 

 silicates of alumina, iron, and lime ; the first compound 

 alone is alterable by the action of water, and is that which 

 plays so important a part in the setting and hardening of 

 cements. 



Analyses of four different varieties of cement support the 

 view that this substance is essentially a tricalcic silicate. 



There is no free lime in Portland cements of good 

 quality, though there may be aluminates and ferrites. 



Drawings taken from microscopic thin sections serve 

 to illustrate the appearance of hydraulic cement when 

 anhydrous and when undergoing hydration. 



Lastly, the author deals with the causes of the destruc- 

 tion of hydraulic mortars in the air, in fresh and in salt 

 water. W. N. Hartley. 



ELEMENTARY MICROSCOPICAL 

 EX AMI N A TION. 

 Elementary Microscopical Examination, By T. Charters 

 White, M.R.C.S., late President of the Quekett Micro- 

 scopical Club. 104pp. (London: Roper and Drowley.) 

 THE author of this work tells us that he has 

 aimed at leading " the possessor of his first micro- 

 scope into the smooth path of progress, by pointing out 

 the simplest and most elementary methods of observa- 

 tion, and, after so far clearing the way, leading him 

 gradually to the higher branches of microscopical manipu- 

 lation." It must be admitted that he has succeeded in 

 doing this. His modest little volume is both sound and 

 original, and confirms the conviction that popular treatises, 

 to be of good effect, must be produced by those who have 

 themselves endured the drudgery of routine work and who 

 have acquired their experience first hand. One sees 

 throughout this work traces of a generation which is past, 

 but as the book is not written for the schoolmen of 

 to-day, criticism is, from their stand-point, disarmed. 



The little volume is, notwithstanding, one of consider- 

 able merit. There may be cited, as bearing testimony to 

 the care with which the author has selected his recipes, 

 the incorporation of the glycerine-alcohol method of pre- 

 paring delicate tissues, originally introduced by Stras- 

 burger. As evidence of originality, we may cite the 

 following : " It (glycerine) needs discrimination in its 

 use, as it cannot be employed for calcareous tissues as 

 bone or shell, as they would become decalcified after 

 being exposed to its influence for some time." Hints 

 such as these, which are the very salvation of the dilettante, 

 can only be the outcome of prolonged practical experi- 

 ence, and they testify most powerfully to the intrinsic 

 merit of the work in which they appear. In dealing with 

 photo-micrography the author describes an apparatus 

 designed by himself. He is evidently an expert in this 

 field, and in his device he has aimed at producing a 

 machine which may be constructed by the worker at a 

 minimum cost. A very worthy motive this, but experi- 



