PRODUCTION OF SULPHITE PULP FROM SPRUCE. 9 
fed in through a U-tube, the receiving end (which was about 5 feet 
higher than the top of the cylinder) having a large lead dish attached, 
into which the sulphuric acid was poured. As the sulphuric acid 
mixed with the sodium bisulphite, pure sulphur dioxide was liber- 
ated. This was passed through a Woulf bottle, containing a little 
water as a trap to catch any sulphuric acid which might boil over. 
From here the gas entered the absorption tank. As the gas pro- 
duced was pure, sulphur dioxide solutions of any strength desired 
could be produced. 
Making the cooking liquor was not so quick or easy, nor was it so 
simple and convenient with the gas as with the sulphur dioxide in 
liquid form, so that the liquid was again used as soon as a supply 
was once more available. 
METHOD OF COOKING. 
The method of cooking received a great deal of consideration, as 
one variable, the pressure, was hard to control. The ordinary method 
of cooking by direct steam and relieving sulphur dioxide at the top 
is unsuitable for experimental cooking because, first, direct heating 
causes too much condensation of steam in the digester with conse- 
quent dilution of the cooking liquor and, second, relieving SO, at 
the top would have a tendency to destroy the effect which we were 
trying to study. For example, in studying different strengths of 
_ cooking liquor, there would be no object in starting with a strong 
liquor, only to blow all the sulphur dioxide out of the digester in a 
short while. Therefore it was decided to heat the digester by means 
of indirect steam, that is, by means of a lead coil placed in the bot- 
tom of the digester; and no gas was allowed to escape until the cook 
was finished. While this method of cooking differs from that used 
in commercial practice, it will brmg out more than any other the 
factors under investigation. 
The method of procedure in making a cook was as follows: The 
chips, the amount of which had previously been determined by 
means of a bone-dry sample, and which remained the same for every 
cook, were put into the digester and 63 gallons of cooking liquor in 
each case were then run in and the digester tightly closed. The 
steam to the coil was then turned on and the cook started. The 
cooking curve for temperature was the same in each case, taking 
two hours to reach 100° C., and three hours to reach the maximum 
temperature, where it was held until it was finished, the pressure 
being allowed to go where it would. Keeping the full strength of 
the liquor to the finishing point in this way gave a hard pulp, which 
had the characteristics resulting from beating for a long time. 
The curves for all cooks are shown in figures 3 to 7. In each 
figure, curve 1 shows the way the temperature was increased and 
14646°—18—Bull. 620-2 
