I dont give a damn!

The following is a real problem statement from the book Chemical Reaction Engineering by Octave Levenspiel.

Chapter 5 ; Question no.28.
HOLMES: You say he was last seen tending this vat . . . .
SIR BOSS: You mean "overflow stirred tank reactor," Mr. Holmes.
HOLMES: You must excuse my ignorance of your particular technical jargon, Sir Boss.
SIR BOSS: That's all right; however, you must find him, Mr. Holmes. Imbibit was a queer chap; always staring into the reactor, taking deep breaths, and licking his lips, but he was our very best operator. Why, since he left, our conversion of googliox has dropped from 80% to 75%.
HOLMES (tapping the side of the vat idly): By the way, what goes on in the vat?
SIR BOSS: Just an elementary second-order reaction, between ethanol and googliox, if you know what I mean. Of course, we maintain a large excess of alcohol, about 100 to 1 and . . . .
HOLMES (interrupting): Intriguing, we checked every possible lead in town and found not a single clue.
SIR BOSS (wiping away the tears): We'll give the old chap a raise-about twopence per week-if only he'll come back.
DR. WATSON: Pardon me, but may I ask a question?
HOLMES: Why certainly, Watson.
WATSON: What is the capacity of this vat, Sir Boss?
SIR BOSS: A hundred Imperial gallons, and we always keep it filled to the brim. That is why we call it an overflow reactor. You see we are running at full capacity-profitable operation you know.
HOLMES: Well, my dear Watson, we must admit that we're stumped, for without clues deductive powers are of no avail.
WATSON: Ahh, but there is where you are wrong, Holmes. (Then, turning to the manager): Imbibit was a largish fellow-say about 18 stone-was he not?
SIR BOSS: Why yes, how did you know?
HOLMES (with awe): Amazing, my dear Watson!
WATSON (modestly): Why it's quite elementary, Holmes. We have all the clues necessary to deduce what happened to the happy fellow. But first of all, would someone fetch me some dill?
With Sherlock Holmes and Sir Boss impatiently waiting, Dr. Watson casually leaned against the vat, slowly and carefully filled his pipe, and with the keen sense of the dramatic-lit it. There our story ends.
(a) What momentous revelation was Dr. Watson planning to make, and how did he arrive at this conclusion?
(b) Why did he never make it?