igo6j Nature Study No. 34. 47 



as it was for immediate use ; the bags weighed ninety pounds and 

 four ol them constitute a barrel. Some of the gravel was hauled 

 by men who received $3.50 a day for themselves and teams; being 

 near the pit, eleven loads were hauled in a day's work ; where the 

 road was good and the haul out of the pit not difficult, a wagon 

 box of one and one-half cubic yards' capacity was used. An esti- 

 mate of the cost is made at a rate of 12 cents a square foot, 

 although this particular walk however was built by day labor. 

 285^ ft. length of 4 ft. walk = 1.142 sq. ft. 



17 > " 9i" " = 229! " 



10 " 6)/ 2 " = 65 



Total, 1 ,436! sq.ft. at 12 c. - $172-37^. 



These measurements were made with a tape line ; by "step- 

 ping-off" the length, and averaging one's pace, a close approxi- 

 mation of the actual cost was reached. 



We afterwards secured some of the cement and examined for 

 fineness, alkalinity, effect on skin, etc. Tests were made, too, of 

 the strength of mixtures of different proportions. Some success- 

 ful object and man modelling was done with it by some of the 

 students. And in this connection it might be suggested that its 

 use is so simple that some repair work on broken walls might be 

 instituted in some schools as a legitimate Nature Study lesson on 

 cement. 



A word on the chemical constitution and action of Portland 

 cement might be of interest and use. In general terms it is a 

 combination of lime (CaO), silica (Si0 2 ), alumina (A1 2 3 ). The 

 lime is furnished by marl and the other two by clay. For good 

 setting qualities certain proportions are essential : 55 to 63% lime; 

 22 to 25% silica ; 7% alumina. Sufficient and no excess of lime 

 to combine with the other ingredients is the desideratum. Water 

 permits the union and crystallization. In a simple form of equa- 

 tion it might be represented thus : 



Base. Acid. Salt. 



CaO (Lime) + SiO., (Silica) = CaSiO s (Calcium Silicate). 

 CaO (Lime) + A1 2 3 (Alumina) = CaAl 3 4 (Calcium Aluminate). 

 So that the artificial stone substance is a mixture of calcium, 

 silicate and aluminate. 



In the last report of the Bureau of Mines, part I, recently 

 published by the Department of Lands and Mines of Ontario, 

 there is a very complete account by Mr. P. Gillespie of the cement 

 industry in this province. Some facts are here included from that 

 report, not for the purpose of informing teachers of matters to be 

 retailed to children, but rather to awaken interest in this line of 



