314 Progress of Foreign Science. 



ceptibly hydraulic ; when the clay is present in the proportion 

 of 15 to 20 per cent., the lime is very hydraulic ; finally, the 

 lime sets instantly, and may be regarded as Roman cement, 

 when the limestone contains from 25 to 30 in the 100 of clay. 

 He considers the iron and manganese as useless towards the 

 hydraulic effect. To appreciate the qualities of a limestone, 

 relative to the kind of lime which it can furnish, it is suffi- 

 cient to determine the quantity of alumina and magnesia which 

 it aftbrds. 



10. Observations on Mortars. 



In a mortar which owes its solidity to the adhesion of the 

 lime to the alloys, (the substances mixed with the slaked lime,) 

 there is evidently an advantage in multiplying as much as 

 possible the surfaces of contact, and consequently in employing 

 a pulverulent alloy ; but, the mortar in that case requires a 

 larger proportion of lime than when we take a granular alloy. 

 On the other hand, the alloys with large grains do not afford 

 mortars so solid as the pulverulent alloys, because there remain 

 among the grains of the alloy spaces filled with pure lime, 

 which do not present the same resistance to fracture as the 

 parts occupied by the alloy. It thus appears evident, that to 

 obtain with the smallest possible quantity of lime, mortars 

 which shall possess the maximum of solidity, we must employ 

 alloys which contain particles of different sizes and pulverulent 

 parts, avoiding always the mixture of argillaceous substances, 

 which can form a paste with water, and which of themselves 

 possess no coherence. M. de Saint-Leger made last 'summer 

 trials on the great scale, the results of which coincide perfectly 

 with these views. He found, contrary to the common opinion, 

 that the sand usually employed at Paris, gives a better mortar, 

 when it is merely washed, than when the fine particles are 

 separated by means of a sieve. 



The pozzolanas, both artificial and natural, differ extremely 

 in their composition ; they resemble one another only in the 

 power they possess of absorbing water without softening ; a 

 power due to their porosity. It is probable, therefore, when 

 they act on lime in a peculiar manner different from other 

 alloys, such as quai'tzose sand, pounded glass, ^c, it is to 

 their porosity, as M. John imagines, that they owe this pro- 

 perty. The important observation made by M. Vicat, that 

 clay slightly baked is an excellent alloy, whilst the same sub- 

 stance strongly calcined is a very indifferent one, supports the 

 same opinion ; for clay slightly baked, and that strongly cal- 

 cined, differ from each other only in this, that the first is light, 

 porous, and capable of absorbing water, whereas the last has 

 become compact and altogether similar to a stone, by the effect 

 of its contraction, which the high temperature has caused it 



