ft 
142 
Transactions of the Boyal Society of South Africa. 
In grains In parts per 
per gallon. thousand. 
Sodium chloride 
Sodium sulphate 
Sodium carbonate . . . . 
Magnesium carbonate 
Calcium carbonate 
6-35 -091 
6-76 -097 
6-68 -095 
16-54 -236 
8-87 -127 
Here again, as in the soils and also in the efflorescence taken from the 
surface of the soil, magnesium compounds form the chief ingredient. 
Sodium carbonate is present in small quantity, the amount being 
apparently less than in the water from Thebus dam. 
The latter yielded the following results in grains per gallon : — 
The water from the dam contains but small quantities of lime and 
magnesia, so that in all probability the carbon dioxide in it was combined 
with soda, forming sodium carbonate, and the use of it, unless accom- 
panied by effective drainage, would therefore tend to increase in the soil 
that most injurious form of alkali, ''black brack." The water from the 
pit, on the other hand, had most of its carbon dioxide combined with lime 
and magnesia. 
On p. 266 of King's Irrigation and Drainage " will be found a table, 
compiled from Bulletin 29 of the Oklahoma Experiment Station, p. 4, 
showing the composition of alkaline waters that may be regarded as safe 
and those which are considered unsafe to use for irrigation. 
Bearing in mind that sodium carbonate represents what is called 
"black alkali" in King's tables, and that sodium chloride and sulphate 
together constitute " white alkali," it will be seen that the amounts in the 
pit water are as follows : — 
Black alkali '095 part per thousand. 
Now, it is to be noticed from the tables in King's book that a water 
may be considered safe if the black alkali — which is the most harmful 
constituent — does not exceed, say, 400 part per thousand, provided the 
white alkali be not too high, but that if the former exceed the figure just 
mentioned the water cannot be regarded as safe. White alkali is less 
injurious, and up to about -500 part per thousand may be passed, but 
beyond this limit, even though the black alkali may be low (as in the case 
of sample 741 of the tables alluded to, where the black alkali is -026 and 
the white -818 per thousand) it would be risky to make constant use of 
the water. 
Total salts 
Sodium chloride 
Carbon dioxide in combination 
45-2 
6-3 
12-8 
White alkali 
•188 
5 ) 
