Ah, cette Line

I've thought that "Ah, cette Line" might be acetylene. But why?

Possible link to electric & water motifs:

In 1895, Henri Moissan discovered that calcium carbide and water produced acetylene gas, and burning acetylene produced light. For the next ten years, acetylene producers flourished until the lower cost of electric and coal gas lighting collapsed the acetylene market.

Possible links to poison/narcotic/murder motifs:

ACETYLENE IS A SIMPLE ASPHYXIANT, IRRITANT, AND ANESTHETIC.

AS A NARCOTIC GAS OR INTOXICANT CAUSES HYPERCAPNIA (AN
EXCESSIVE AMOUNT OF CARBON DIOXIDE IN THE BLOOD).

ACUTE EFFECTS: SYMPTOMS SUCH AS HEADACHES, DIZZINESS,
SHORTNESS OF BREATH, AND LOSS OF CONSCIOUSNESS MAY OCCUR IF
THE GAS IS PRESENT IN QUANTITIES SUFFICIENT TO DILUTE THE
OXYGEN CONCENTRATION IN AIR. SYMPTOMS OF ANOXIA(ABSENCE OF
OXYGEN).

There also seems to be a chemistry motif in Ada. There is at least one chemist in that family tree & I thought a few times I spotted a possible reference to Rimsky-Korsakov (chemist & composer) in the novel.

Carolyn