Sentence:  Following observation of the fact that the reaction rates of supposedly identical reaction mixtures prepared on the same filling manifold and exposed under identical conditions often differed by several hundred <WSD>per</WSD> cent , a systematic series of experiments was undertaken to see whether the difficulty could be ascribed to the method of preparing the chlorine , to the effects of oxygen or moisture or to the effect of surface to volume ratio in the reaction tubes . 
Meaning of the word marked with <WSD> tags:  a proportion in relation to a whole (which is usually the amount per hundred)