Having established that Herbert was gassed at the Second Battle of Ypres in April and May 1915, the next step is to try to identify the actual attack. It has already been established that Herbert almost certainly served with the 27th Division, and possibly with the 19th brigade or 67th battery of the 20th brigade. A search of the 27th Division's artillery war diaries is therefore required for April and May to look for a possible gas attack.
Although it is the 19th brigade and 67th battery that are of most interest, all the artillery brigade war diaries of the 27th Division are checked for completeness.
Each of the three artillery brigades will be discussed in turn.

Men of 2nd Batt. A&SH wearing early gas masks consisting of goggles to protect the eyes and a pad impregnated with chemicals over the mouth. (Photograph courtesy of the Imperial War Museum, London - Q 48951)

1st Brigade
The 1st brigade war diary is not particularly detailed and non of its batteries kept war diaries. There is however a typed copy of the 1st brigade commander's contribution to Brigadier General Stoke's (CRA) papers "Recollections of Ypres 1915".

The war diary states for 20th April "Germans shelled whole area using new chemical which made eyes smart and had choking effect". In "Recollections of Ypres" it is stated that tear gas was used on 24th April. As far as casualties were concerned it is just stated in the war diary for the period 20th April to 27th April "During all the above period the wagon lines & communications through Ypres were heavily shelled and many casualties caused".
On 24th May the war diary states "Germans attacked at 2.45am with gas and heavy artillery all along front from Menim road to Railway......". Casualties stated as 2 wounded and 1 killed. Recollections of Ypres states "...The gas was very trying as we had only indifferent respirators".
It is possible that the 1st brigade suffered up to 2 gas casualties on the 24th May if the commander listed gas poisoning as wounding and maybe some gas casualties towards the end of April, but the diary is very vague during this period.

19th Brigade
All the batteries of the 19th brigade kept separate diaries (although not for every month). The 19th brigade war diary itself is of average quality.

For April, the 19th brigade war diary makes no mention of gas. For May, it names all the brigade's casualties. Herbert Halliday is not mentioned and neither are gas casualties. There is only the smell of gas mentioned on the 24th May. It therefore seems very unlikely that the 19th brigade suffered gas casualties during the Second Battle of Ypres.

20th Brigade
Both the 67th and 364th batteries of the 20th brigade kept separate war diaries. The 20th brigade war diary itself is of a high quality, as are those of the individual batteries.

For April, the 20th brigade war diary makes no mention of gas. The entry for 2nd May states that at 5 p.m. "Northern observing officer reported that yellow gas fumes were being ejected toward our trenches.....". At 5.10 p.m. "67th Bty very heavily shelled..... Casualties (for brigade) 1 killed and 11 wounded". There is no other mention of gas in the 20th brigade war diary for May. At the end of the month the war diary states "Total casualties for the month of May:- Officers - 1 killed, 2 wounded. NCOs and men 2 killed, 27 wounded, suffering from effects of gas - 2 men".
The 67th battery war diary sheds more light on what happened to those two gas victims. For the 2nd May the war diary states "..... In the wagon line 2 drivers were wounded and at the forward observing station 2 gunners were partially asphyxiated by poisonous gasses from hostiles shells & bombs and admitted to hospital".
It would seem that the 20th brigade suffered two gas casualties during the Second Battle of Ypres and they were incurred by the 67th battery on the 2nd May. It is also interesting that the 20th brigade only lists wounded on this day. If the total number of wounded that were reported in the 20th brigade war diary for May are added up, they come to 2 officers and 29 NCOs and men. Yet at the end of May they are reported as 2 officers, 27 NCOs and men and 2 gas casualties. This clearly indicates that gas casualties can also be reported as wounded.

What happened to Herbert Halliday?
From the medal rolls it was shown that Herbert disembarked on the date that the 19th brigade and 67th battery disembarked. The 19th brigade appears not to have suffered gas casualties at the Second Battle of Ypres, however the 67th battery suffered two gas casualties, the only gas casualties suffered by the 20th brigade. The situation regarding the 1st brigade is a less clear.

Did Herbert serve with the 67th battery and was he gassed on the 2nd May 1915 at a forward observing station? This seems unlikely. According to the casualty list in the Times he held the rank of driver when gassed, yet the commander of the 67th battery states that 2 gunners were gassed. It could be that Herbert's rank as listed in the Times is incorrect, but again this seems unlikely as there is other evidence that he was a driver. If Herbert wasn't gassed while serving with the 67th battery, we are just left with the vague references to gas in the war diary of the 1st brigade. But Herbert disembarked the day before the 1st brigade disembarked. Or did he?