This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
187. To build any kind of a brick structure, so as to make a strong and durable piece of work, it is necessary to have a bed of mortar between the bricks. Brickwork consists therefore both of bricks and mortar. The strength and durability of any piece of work will depend on the quality of the brick, the strength and quality of the mortar, the way in which the bricks are laid and bonded, and whether or not the bricks are laid wet or dry.
188. The function of the mortar in brickwork is threefold:
1. To keep out moisture and by filling all crevices to prevent as far as possible excessive changes in temperature.
2. To unite the bricks into one mass.
3. To form a cushion to fill up any irregularities in the brick, and distribute the pressure evenly.
The first object is best attained by grouting or thoroughly flushing the work; the second depends largely upon the strength of the mortar, and the third is affected chiefly by the thickness of the joints.