Sentence:  If, on the other hand, we remember that a <WSD>tertiary</WSD> monoamine, such as must be formed by the final methylation of the ammonia fragment in aniline when submitted to the action of an alcohol chloride, is invariably converted into an ammonium compound it must appear rather strange that in the process above alluded to only <WSD>tertiary</WSD>, and never any quartery bases are observed. 
Meaning of the word marked with <WSD> tags:  Possessing some quality in the third degree; "especially" having been subjected to the substitution of three atoms or radicals.